BT and Be* do not give "fixed caps" in their FUP,
BT state its subject to "Network Management" (same as plusnet)
BT / Be* 'reserve the right to remove you if you use too much' (at their discretion, an amount isnt given)
However... O2 (who resell Be*)
o2 access is 20gb, o2 basics is 20gb, o2 100gb for the allrounder, and 250gb for the works
Which personally ive exceeded all of those many times.
TT is a painful company, you shouldnt even compare them to any ISP other than maybe Orange.
Tiscali customers (owned by TT) their FUP states 40gb is "typical" so id expect the limit to be 60 or 80gb
TT does traffic shaping (who doesnt these days? and before you say O2, do some digging - they do but its not the same style of shaping) Tiscali/TalkTalk do not distinguish between time of day, so if you get poor speeds due to you using too much on PN, you will find you probably end up with poor speeds 24/7 with TalkTalk - then go over the amount your allowed to use, and get stuck in an 18 month contract :)