Well you either want BT which is:
40gig/month for £29.99
20gig/month for £24.99
and 'up to' 8mb which you may get close to!
Or you have unlimited NTL:
£24.99 for 'up to' 4mb
£34.99 for 'up to' 10mb!
which you may/may not get full speed on depending on how many pr0n leechers/warez kiddies are in your area now it's an 'unlimited' service
So it's really a question of what's more appealing to YOU!?
As for the always on bit, just get an ADSL router that redials dropped connections and let that maintain an 'always on' connection for you.