In the UK you could certainly find somewhere good to base yourself that would provide good access to University and top event riders.
The Universities of Bath and Bristol have good reputations and are surrounded by the UL Gloucestershire event contingent. It’s also within reasonable striking distance of Talland, one of the riding schools here with a better reputation.
Your problem will be the short time period and finding access to a horse to compete. It doesn’t work like the US where you just seem to pay to lease a horse based at a riders barn. Private leases or loans are available but no private owner is going to do that for an unknown for such a short period.
If you based yourself on an event yard as a working pupil and prove yourself there are yards out there that would support you in finding access to something to compete. But 3-9 months isn’t long enough - you will only just be proving yourself by then, and that is in a working situation anyway.
I don’t know what the situation is in continental Europe but I can’t see it being too different, and they don’t have the same access to the volume and proximity of events that the UK does.
Other options that could work - our universities run sporting teams, one of which is riding (BUCS) - it would be a good way to meet people and potentially compete. You use riding school horses rather than personally owned ones. The standard is hugely variable (riding and quality of horses) and is only combined training (dressage and sj), basic league stuff starts at 80/90cm with an easy dressage test. But the good teams go through to championships where the standards are higher and fences a bit bigger.
If you are a good and experienced rider you might also pick up a ride locally if you advertise your time or are willing to pay - Facebook horse groups are a good place to start. Again, finding a good quality horse might be tricky but not impossible. You could then invest in freelance training, which is far more common in UK and how most normal people do it. Very few (only really aspirational young riders) tend to be based with a trainer