How far are you willing to travel? I'm in Kent and know quite a few, but depending where you are in Surrey I suspect most will be about 1-1.5hrs which is quite a way!
Pachesham - advertised on their own site
Rose Hill - often have PC/RC ODE's running - check their website for which branches to find the schedules
NFED website's calendar often have a lot of the PC stuff listed on it as well.
Tweseldown - great course, running a series of unaffiliated with Crown farm and another this year.
Edit:
Snowball farm (the old Burnham beeches BE course) often run stuff throughout the year
Ok, you may have to search the distances for some of these, but hopefully some of them are within a reasonable distance for you.
Bonfleur - near Maidstone, have a lot of things on, there's a calendar on their website. I know there's an ODE there on 25th June (it's pony club but also open to non-members)
Tweseldown - calendar on their website, they're running an unaffiliated ODE every month until September
Munstead - not sure if they're running any more unaffiliated ones, but they do run them occasionally
Brightling Park - running an unaffiliated in July, on FB or EquoEvents (NOT on their website)
It's worth having a look on your local pony club websites, a lot of their ODEs are open to non-members. You'd probably be in area 13 (though some events in 11, 12 and 9 probably wouldn't be too far, I'm area 11) You can just search "area x pony club dates" and then find any that are interesting to you!