Hi
Bonus are good and can be quite flexible but can be busy at times so you'll need to book ahead to get the slots you want.
If you're looking in the Cranfield area can I suggest you have a chat with David Coulson at Cranfield Flight Training (I think thats the right name...) - they are based near Bonus (just past the cafe pacific and CATS).
David is probably the best instructor I have ever flown with and has a massive wealth of experience to share. He was absolutely great when I did my FI rating with him many years ago and even cured my fear of spinning in the PA38. He and his team are very flexible, most helpful and possible a little less expensive than most other schools. I believe they currently use Seneca for MEP/IR but also have access to a Twin Star if you might prefer a 'glass cockpit'.
As an alternate, there is always Stapleford - used to be a great school when I started out (about 12 years ago) and I've only heard good things since.
Good luck,
Ian.