"i was under the impression that you could only claim VAT back to the amount that you have invoiced for ....."
Not so. if you have more inputs than outputs, HMC&E can send the difference back to you by BACS (ie immediate bank transfer) on the due date. I had this when i set up a business in a different sphere and had up-front costs to reclaim.
The next issue to face is whether the people you will eventually work for don't mind paying VAT on your services. If you remain self employed, there's also the small issue of IR35, but let's not go there on a sunday evening.
Time limits are also not helpful.... depending where you are on the training.