Hi Maxfella,
NO is the answer, you must go through a boa-fide Training Centre or College. All assessors and assessments must adhere to a strict quality procedure (Internal and External Verification) to ensure the validity of assessment.
The term 'get signed off' infers you do not understand that you will undergo on site assessments. You should be 'working' with a qualified plumber, doing real work to industry standard. Therefore the fees of £1k to £1300 would appear to be fear and indeed the going rate.
I do not wish to put you down or deter you from a career in Plumbing. However, the Full Time College courses scenarios are unrealistic and do not prepare you the 'real world'. No short cuts just hard work.
Regards
Teabreak