Circuit Days have Donington on 16th March for £219, 7th August for £229 or 8th Nov for £169.
Bookatrack and Javelin have a host of dates across the year ranging from £199-£299.
I prefer to book Open Pit Lane events. It means you can come and go as you please rather than having 20 minute timed sessions where you are allowed out on track.
Most track days you'll need to pay extra for extra drivers and passengers so just take that cost into account too.
Everyone should experience down Craner Curves flat out though. It's a great piece of track.