It depends.
If you have £400 just for the rifle, then as
@Musica says, the SRS sport is an excellent choice.
I have one and it's a lovely piece of kit. But, needs upgrading to get the best out of it.
Alternatively, for a little more (£444.50 delivered) you could get the SSG24, which should not need any upgrades. One of my team mate runs one and it can compete with my SRS.
Any other base rifle you'll need to seriously upgrade yourself, so it depends on how you feel about that.
An A&K M24 can be had for under £100 and needs little more in the way of upgrades than barrel/bucking and spring, as everything internally is already steel and so doesn't need upgrading.
Or you can get a VSR/clone and just upgrade the lot...
Like I said - it depends ?