Unfortunately not.
All CPL training must be done at an approved school. The chances of an approved school letting you use your own aircraft are next to nil.
The aircraft for a Commercial skills test has to also be approved for the purpose on a form called the FCL 176 which isn't cheap. This is specific to each aircraft registration so given the financial constraints you wouldn't be able to use your PA44 for the test.
There are UK CAA FIs at all UK FTOs. EFT uses all JAA FIs with FAA conversions. OBA uses a mixture of both. Can't speak for the other schools.