If the TV doesn't have some kind of input for RGB, well, you would have to mod the TV. I remember hearing there were places that could do that for you, but now, I'd guess you'd have to learn how to do that yourself.
Your more feasible option is a component video mod for your PC Engine and the fact that most modern TV sets have analog component inputs. There are plans here by our modding experts to sell boards that do it once thesteve gets his inexpensive design ironed out.
And yeah, if you bought one of those PC Engines that were modded to output a PAL signal and sold in the UK, as opposed to the usual NTSC standard, then that is a problem. I would guess you would have to get around the mod, should be possible. I don't think they changed any actual chips with those systems, so tapping the RGB off the 6260 chip would be in NTSC.
Correction: The RGB state of the video signal is prior to any NTSC/PAL encoding, so that doesn't matter.