I have a mate that is doing his CPL and IR part time whilst working. It really is a struggle for him. Thing is on paper it looks like a great idea. BUT what happens in practice is that on the days/weekends off you have planned to fly things like weather, instructor availability, aircraft serviceability, airfield closures etc etc all come into play. Then depending on your own ability you might find that with a bit of a break between lessons that you have to go over old ground which will amongst other things add to the costs. I can tell you from experience at a PPL level that the longer it takes you the more chance you have of your instructor going AWOL which means you start again and are going over old ground with someone new.
Whereas if you are sitting around a school day in and day out then generally they will try their darnedest to get you up in the air after any protracted delays. Sometimes maybe flying twice a day if they have an aircraft available.
Other thing to consider is the great British weather. What month you start your training (more so for CPL but CB's, howling winds and turbulence aren't what you want for the IR) can have a big influence. I started my CPL/IR back in September of last year. I got through the CPL in about 4-5 weeks. Some guys starting after me weren't so lucky and were grounded for a couple of weeks which didn't help matters. The IR can race along at the start as depending on where you go part of the course can be done in an FNTP2 so no weather issues to take account of.
In summary my advice would be to try and do at least the CPL full time and get some more holidays. Then have a break and go back to work if necessary and then do the same for the IR. Failing that if you must go part time then make sure that you find a school that buys into what you are trying to achieve. More than likely it will be a smaller school I am guessing who might have an instructor that is willing to work weekends.