I gained a Canadian PPL and had no problems using it in the UK on my return (apart from the fact it's difficult to get your head round the enormous cost differential!).
You don't HAVE to convert to a UK license to fly here. This would involve sitting all the written papers again - not sure about a flight test as a Canadian PPL is ICAO standard and you might be Ok.
I was able to fly with my local flight school after sitting a UK air law exam and one of the flight school's own papers. I was current on type but found I needed several hours of conversion just to get the hang of the hugely different R/T (buying a scanner may help!), and the differences in proceedure.
Hope that's of some use, sorry I have no information on how different ratings (multi, etc.) fare in a transatlantic conversion, although I don't forsee any issues other than currency with the night rating I hold.