Firstly, multi time during hour building: By all means, do some if you want - it's great fun! However, I would say don't die in a ditch over getting it in. You will find that it probably doesn't make a huge amount of difference in how many hours you take by the time you have finished.
One of the guys I went through the CPL/IR with had done his MEP rating in the States and I think it took 4 or 5 hours off the minimum requirement for licence issue, but not sure if he actually found it an advantage (I remember him saying afterwards that he would rather have done the additional ME hours during the CPL course, so he would have been more current/comfortable come the test - currency is everything!).
I would also suggest that you go out and use the hour building to try new things, as well as improving your skills. If you can, do some IF, aerobatics etc. It will make you a much better pilot overall (and help you get through the courses a bit more easily IMHO). I did an IMC rating, AOPA Aerobatics course and a formation flying course during mine and it was bloody good fun! As Mikehotel152 says, enjoy yourself! It will be the only time in your life that you have to go and do a load of flying - and what you do is almost entirely up to you!
It is not difficult to practise the CPL skills while also going to fun places with your mates - all you need to do is practise the skills you will need while doing it (and should have already learnt on your PPL anyway!): Plan on a chart, not SkyDemon etc., don't just follow the GPS, hold yourself to good standards of accuracy in your nav and don't just get lazy. If you can, as already suggested, do the occasional trip with an instructor to make sure you aren't picking up any bad habits. Simple!
Just remember, the hour building is designed to give you the experience and skill level necessary to complete the CPL. Too many people see it a just a number that has to be achieved, regardless of what they do. Trust me, you can tell the difference between a pilot who has just burnt holes in the sky for 150 hours and one who has done something useful with their time...
Don't forget though, that you need 100 hours PIC as well as just 150 total...
A final suggestion would be to save some of your flying for the UK. Having come across a lot of US-trained students while instructing (and having discussed with many instructors who teach at all levels), it is obvious that flying in US airspace doesn't fully prepare you for flying in the crowded UK, with its different procedures and ATC etc. Well worth doing a proportion back here (probably the last remaining flying before you start the CPL).
With regard to the ATPL theory, the previous comment about concentrating on that before the majority of the flying is a good one. I would suggest that most of it is not hugely complex, but it is the sheer volume of material to wade through that makes it "difficult"! What I used to do was "schedule" myself that I could only go flying once every x hours of studying/y progress tests completed etc. It didn't always work(!) but did help to keep things moving most of the time!
Good luck and enjoy yourself!