IT would be more than£15k as you'd get tax relief on your contribution at 40%. So if you put in £7.5k you'd have £3,000 refunded by the IR. So in effect you'd be putting in £18k.
BALPA have excellent IFA's (Independent Financial Adviser) who will help you understand this much better than I can explain it!
As for what your pension would be worth well speak to an IFA.