One more thing to mention - the ATPL exams will include the Instrument Rating theory, while the basic CPL will not.
Depending on where you are thinking of ending up, an IR may be important.
I did mine with Bristol GS some things to note
1) Their course (as of 6 months ago) contained a lot of fixed wing specific information (e.g. the FMS on 737, how to navigate across the Atlantic using NATS etc). All interesting stuff btw! but some people found (or perhaps better, made) it hard going since it was easy to discount the information as not appropriate for helicopters.
2) Distant learning is a hard slog. I did it full time (i.e while not working) and it was still a lot of work and effort. For the people there who did it while working, for the most part they took around 2 years to get it done (and I suspect there was a fair percentage of "dropouts" - especially those with other commitments at home)
3) The way the course works, you do the studying yourself and get 2 weeks "refresher" (or cramming if you've been slack in the studying). I did it having about a 1000 hours of mainly instructing time, with some other flying (including fiuxed wing), plenty of flight simulator time and having read around the subject a lot.
However, there were those who turned up with a PPL and minimum hours who had even less exposure to aviation (particularly the fixed wing world) than I had and it was an up hill struggle for them.
Finally, there's no way of avoiding it, 2 weeks in a class room to revise ~8 subjects is not a lot of time. If you think you might be a bit slower than average at picking up the information, you may well find it difficult to get everything covered in such a short period.
BG