Since you're saving exactly £500 per month, I'd recommend opening a regular saver account. With Coventry, you can deposit up to £500/month. Regular savers are great because they offer higher interest rates than other savings accounts. This page 15 has a list of the highest-paying regular savers atm.
I won't recommend putting the money in a stocks and shares ISA with only 3 years to go before you buy the house. It's just too risky.
I hope this helps!