Can't seem to post on Yahoo so...
If you can fly Thursday or Friday you should as the weekend looks overcast and rainy.
Tomorrow Thurs will start cold at 2C (cover your veg) and will rise to 16-18C with good climbs and cloudbase and light NNE'ly. North and East of Northampton will be affected by N Sea air which will lower temperatures and clag up. The SW will be hottest 23C but is expected to go blue.
I think that 500+ is on but in a limited task area. Stay below a line from Snowdon to Northampton. The wind is not strong enough for ridge lift on the South Downs but there will be plenty of climbs down there. Go West as far as you like but expect blue.
I have'nt looked at Friday thoroughly yet but it should be OK for XC.