I don't think its worth it. You'll get mixed opinions on this.
I'll give you a scenario...
Let's say in TOC A you sit the following tests;
- Group Bourdon
- TRP 1 & 2 (glop test and cables & dials)
...then the remainder of the tests
Then TOC B may have a different assessment test...
- group bourdon
- TRP 1 & 2... Which turn to be different. Fault finding test and a different test to the glop
Some also require a verbal communication and a mechanical test
So let's say you apply for TOC B after receiving results from TOC A, you will need to sit them additional tests. Regardless if you have passed the whole lot.
Also some TOCs insist you sit the whole thing again, regardless if you have passed the tests. (I believe First group do, although I could be mistaken?)
Whilst you pass the tests at this testing centre, they may not meet the TOC/FOC standards, meaning a borderline would be considered a fail to them. (Not a RSSB fail)
I think its better to just apply to the TOC/FOC and keep doing it. It really is a lottery! Then sit the process there and then. Also jobs rarely come up, I think 3 years isn't long enough so £900 could be a waste. Of course some will disagree with me...