Schoonerboy
I would definitely do your ground school first.
This is not intended to in anyway question your abilities, but it is a fact that some students cannot get through the ATPL theory exams. Sad but true.
To ‘drop out’ at that stage would be bad. To drop out having done a £13k plus IR course would be beyond contemplation!!
You say that you are only recently qualified. Well if I were you I would get as much flying experience under your belt as you can before you start the £300+per hour IR training.
There is no doubt that hours and CPL training will save you money in the long run. You will start your IR training with more knowledge and confidence.
In addition, the ground school does help your understanding of instruments.
Good luck