Jack.
You should be aware that Highlands and Islands Airports Ltd do offer ATCO training, but they use a selection process which almost mirrors the NATS one.
There is a lot of competition from high quality local people and staff at their airports, and although it's not the sole criteria, they are very keen on choosing people from the communities where the airports are, i.e the Islands.
Opportunities do exist to move on to Inverness, Dundee, the mainland etc but you'll need to put in a couple of years before that happens.
As has already been said, for someone with no experience at an airfield, funding your own training is an extremely high risk to take. If you fail, there's no guarantee of getting your money back.
You may want to have another bash at NATS when you are permitted to do so, but until then, a good way of getting atco training is to get a job as an ATSA at a regional airport.
Prepare a short, but concise letter selling yourself with the reasons why you want to get into ATC and send it to the Manager of ATS (find out their name first) at every regional airport where NATS don't provide the ATC services and ask for an opportunity to go and spend half a day with them.
Most regional airports have ATCO training schemes from time to time and they nearly always select from their on ATSAs first, again, competition is fierce and you probably won't be considered until you've had a year or so's experience, but it's worth the wait.
Whatever you do, do your research first and get some interview tips from the forums on Pprune, there's a lethora of information available.