I would say your chances are negligible! A plot just big enough for a detached house in Surrey would cost more like twice the price you set as a limit. Better option is to buy an old place and demolish it as the majority of this green belt land is on a one for one basis. There is a land agency with a monthly newsletter that I subscribed to for a year in my 2 year search for a plot. Problem I found is that all the agents are in the back pockets of local developers large and small who will make a commission on the sale of the plot/site to them and then make another 2% when the builder puts the new house back on the market with them 6 months later. Private builders can't get a look in. Better bet is to find a local small developer in tune with your taste in house building and let him do all the legwork/investment. It worked for me in the end and total cost was about 10k higher on a 350k property than if I had done it all myself, plus I got to sleep a lot more whilst it was being built.
Good luck.
Bob