The UK CAA has approved a course of training (which is not JAA compliant - but they have the rights) whereby the MEP class rating *may* be combined with the training for a JAA CPL.
The published minimum training time for a CPL is 25 hours, the published minimum for an MEP class rating is 6 hours (both require additional time for the resulting flight test for licence or rating issue).
The UK CAA have authorised a combined MEP/CPL course with a published minimum training time of 28 hours which must include not less than 8 hours of multi engine training.....as it says in LASORS (spit).
Their reasoning is that the CPL flight test must be conducted in a "complex" aircraft (minimum 4 seats, retractable undercarriage and constant speed propeller). The training for the conversion from a "conventional PA28 or similar, is so close to that required for an MEP class rating that they have authorised the mutation of the two courses.
As an aside - and only my personal opinion (which is not necessarily that of the UK authorities or OFT) - it would be far more beneficial to you to complete as much multi engine time as your bank manager will allow.
The "best" breakdown of hours that I've seen is that started by Oxford in Arizona, whereby they flew 14 of the 28 hours in a PA28 and the remaining 14 in an MEP (PA34, Seneca II as it happens).
If doing this *at* OFT, as you suggest, you would be looking at the price difference between six hours of PA28 against the same extra six hours of PA44 Seminole. That would be in the region of US$600 - a little over £300.
Not bad at all for an extra 6 hours of MEP time and the greater experience that you present yourself with, at the commencement of a US$2,000 flight test.