I suggest you look at LASORS on the CAA web site, which gives you an idea of what you have to do to convert from PPL abroad to a JAA PPL. The document can be found here:
www.caa.co.uk/docs/33/LASORS.PDF
Basically, the situation is much easier if you come back with more than 100 hours - you then only have to do the air law and human performance exams and take a skills test. You can transfer any night rating, but not sure about a twin rating. It is not a CPL night rating, as it is added to the PPL. The night qualification is however required to get a CPL. All your flying time should count towards the requirements of the CPL and IR. I take it you mean to do the CPL and IR training in the UK? That will be 25 hours CPL + 50 hours multi IR if done in that order or 55 hours IR and 15 CPL if done in this order (you are exempt the 10 hours instrument flying on the CPL if you hold an IR.