You don't need to change your FAA PPL. You can get checked out again and carry on flying in a G reg on your FAA license to build up your hours, providing you stay in UK airspace. The hours you have will still count, or at least a certain % of them will anyway.
Then just do your CPL/ME/IR and they will be issued as JAA licenses, so from then on you will fly on those, so no need to convert the PPL.
If you are giving up work to do the ATPL ground school, you may as well do it as a residential full time course. Much better than distance learning and struggling by yourself, on a full time course they steer you through the mud so you can concentrate on the stuff you really need to know and it is only a few hundred £ more.
