The hardest thing about converting your licence (even considering your experience) will getting to grips with the IR.
Flying IFR in uncontrolled airspace in the UK has lots of little nuances, many of which only become apparent when they actually occur. Experience... it's what you get when you least want it!
The ground exams will take 6 months full time studying or 12 months part time- and that's if you really go for it.
The CPL is no problem. A simple "tick in the box" exercise with very little relevance to flying commercially in the real world (unless flying commercially involves pootling around on a VFR route single pilot in a PA28R). Because you will be converting your FAA CPL, you will simply have to do "training as required", plus 170A, then test. Tip: if you do do it, do this bit in the USA.