There are a few, TrainPal is doing £5 off (I believe £15 minimum, although I'm not sure so would appreciate the info if anyone does?) ticket with a variety of promo codes e.g. from Geoff Marshall or a quick Google of "Trainpal Promo".
Loco2 is doing £5 off £10 minimum if you find someone that can refer you. Both of these are for new customers only and neither of these sites charge booking fees.
There is also the option of cashback sites which can give you a percentage back, Chiltern offers 3.5% back on all tickets with both TopCashback and Quidco at the moment. Loco2 does 3%/1% which might stack with the referral offer. Transpennine offers 4% back on their advanced tickets only (1% on others so use Chiltern otherwise).