To answer your question "but say my rent and/or service charges increases by £25/year and the Housing Costs element rises accordingly, will that decrease my TP by the same amount so that in 6-7 years it will all be eroded and I'll be considerably worse off as a result of migrating from ESA to UC?" Yes unfortunately, that is the intention built into UC, to make savings with Benefits.
David
Nothing on this board constitutes legal advice - always consult a professional about specific problems