Please excuse these disjointed thoughts, but my parents are visiting and — plainly — they come first. I’ll put my ideas to paper, er, blog in this a couple of following posts and hope that comments help fill in the gaps.
To recap: I think that the British Unitarians and Free Christians need to form new congregations to survive. Adding new blood to the existing congregations won’t do alone, assuming the truisms apply in Britain and the United States alike. In short, people are more likely to join a new church than an old one. Perhaps it’s just to much to expect people to align themselves with a centuries-old institution that they have no purchase in. And besides, no institution lasts forever. Some churches must need be formed to replace those that have died.
Now, to consider the rules the British Unitarians have for recognizing new churches. It makes little sense to imagine some ideal mode of church starts — even if I could describe it, which I cannot — if it runs against the instituted rules. Perversely, there aren’t enough church starts to prove the  rules a failure, and the General Assembly of Unitarian and Free Christian Churches (GA) is too newly revised to suggest a regular review.
Thus, to organize a church, let’s review the GA bylaws:
2) Conditions for Membership of the Assembly
2.1 Congregations
2.1.1  Applications shall be dealt with in accordance with the procedure laid down in Clause 5 of the General Assembly Constitution.
2.1.2  A congregation must  have at least 12 subscribing members over  the age of 18 years, and must have existed for regular worship for not less than one year.
2.1.3  A copy of the rules and/or constitution and by-laws must be supplied and these must be approved by the local  district association and by the Executive Committee.
2.1.4 The constitution must embody a clause specifying that, in the event of the congregation ceasing  to exist, its funds and property  shall be transferred to an approved, specific body.  This will normally be the sponsoring district association or the General Assembly, as appropriate.
2.1.5 Meetings for a religious purpose must be held at least once a month.
2.1.6 An annual subscription must be paid to the Assembly and to the district association, if required.
2.1.7  A copy of the annual report and audited/independently examined accounts as submitted to the annual meeting of members must be sent to the district association and to the Assembly.
2.1.8 The application must have the support of the district association.
2.1.9 Before the application is approved the congregation must be visited by a representative of the Executive Committee who shall make a report on the visit.
American (and Canadian) Unitarian Universalists will recognize the shape of these requirements. The requirement of twelve adult members is close to the 10 member requirement that existed in the UUA before the 1990s. (It is now 30, and was briefly 50.) The one-year requirement is a prudent requirement, if informally imposed in the UUA system through its system of deadlines. The “annual subscription” — a term I prefer to the cloying “fair share” of the UUA — is £ 24: less than the UUA’s requirement, but comparable. Likewise, the monthly meetings requirement is close to the 10 times yearly requirement of the UUA. The audit requirement of 2.1.7 is particular to English and Welsh charities law. (I don’t know about Scottish and Northern Irish law, but I suspect it is so there, too.)  District support and a site visit maps to practice in the United States, if not UUA bylaws.
All of which is by way of saying that I think we might have some resources, even if the UUA’s track record of new congregations is itself poor.
But then there’s another option that is both enticing and odd. If a congregation of twelve wasn’t small enough, there’s a this provision:
2.2 Small Congregations These shall be given recognition provided that they shall have been meeting regularly for 6 months.  They shall be admitted on the recommendation of the district association if they comply with the above conditions for Congregations except that the number of subscribing adults shall be reduced to 8 and the requirement for meeting shall be amended from ‘at least once a monthâ€
to read ‘at least bi-monthlyâ€.  Small congregations may send observers to meetings of the General Assembly but without the right to vote.
So, I’m reading this as a recognized provisional membership. Eight adults, six services. Just enough of an institution to rise above the waterline of an informal and casual grouping. On the one hand, I can’t imagine a congregation that meets only every other month would grow or even survive.
But here’s a thought. Leaving fifth Sundays aside — and assuming Sunday worship  — bimonthly services makes eight “slots” out of a full-time schedule, and since afternoon services are common in Britain, let’s say sixteen. Imagine organizing not one new church in an area not too far from other some ministered congregations — say, south London or around Milton Keynes — but several. Even sixteen. Say, associated with every other station on a rail line, or more objectively, about three-quarters the time of the average commute in the target area. Thick, by current standards. And close enough for members of one new church to seek out worship and fellowship at neighboring new starts as they move from bimonthly to more frequent services.
You’d need a corps of lay ministers and perhaps the assistance of one or more experienced, enrolled ministers. The worship schedule would have to be carefully coordinated, but the congregations would be encouraged to take on their own local character. Some would fail, but without the risk of failure there can be little hope of success, and the relatively high density would provide a safety-net for the members of the failed church starts.
WWUUD stream 
.svg){kind=link}