Go Big Early!!
Would stay away from Bonus aviation and go somewhere reputable. Some of their instructors are very good unfortunately some aren't. If you are already using Bristol Groundschool why not use Bristol Flight Centre. They have a very good pass rate and lots of spare aircraft. The amount of flying you need to do when converting is so little that differing institutions make little price difference unless you go to Oxford or Cabair.
In these times you do not know who will be operating next week, go to the big schools with lots of aircraft and on average the best instructors IMHO. You also tend to meet more people which = more leads for job hunting later. Alumni are a very necessary part of the UK job market. In fact I would say that it is more important than your ability as a pilot.
VT