Dunno if this document is relevant
http://www.caa.co.uk/docs/33/CASPEC05.PDF
I use ex-MOD Mk25 life preservers (if they are good enough for the MOD.....) which are serviced by SEMS annually. I also have an (oldish) Satfind 406MHz PLB (registered with MCA).
Re. 406MHz PLB's - don't ever be tempted to buy one cheap from the USA (or anywhere abroad) as MCA will ONLY register PLB's that have been programmed with a UK designator...
This is an 'interesting' problem, particularly flying in the Channel Islands where you rarely get an inter-island transit above 1000' - not much chance to glide in midway there....