As far as my contribution is concerned (sharing what I have used):
The best Economical one (Best for International calls Only, Flexible and fix, suitable as Additional contract):
T-mobile you fix sim only plan
£10.00
30 day (advantage, can cancel anytime)
100 minutes (fairly less with no Text Messages)
unlimited landline talker (no FUP, as I checked but only 2 calls conference)
http://www.t-mobile.co.uk/shop/you-fix-sim-only/plans/The best All Rounder (Best for International, National calls and Texts):
O2 on & on, 12 month
£20.00
Unlimited Minutes (AFAIK FUP 12,000, with upto 6 calls conference)
Unlimited Texts
1gb Data
http://www.o2.co.uk/tariffs/simplicity