Farnborough 1904
Brooklands 1907
Fambridge 1908
Barking Creek 1909
Eastchurch 1909
Leysdown 1909
Blackpool 1909
Acton, Beaulieu and Hendon 1910
I'm not going to stick my neck out and say those dates are definitive but they are the earliest I can find. One problem is that sites like Farnborough and Larkhill (1911) gradually changed from balloon to fixed wing and it is difficult to quote an acccurate date for fixed wing ops when the site is already well established.
Again, does "aviation began" specifically mean fixed wing? If not then ballooning goes back to 1784 (first UK flight) and by 1880 Aldershot was a known "factory" and testing centre. By 1884 a military balloon was used in overseas operations.
If we dismiss balloons what about airships?
In the end the question becomes so definitive that we end up with a string of dates, all equally valid for their specific application..
jb