There are a number of FTO's in your area and you now have a map! You have the right idea with the trial lessons, as for what aircraft, I'd go in the type the FTO uses for training. This could be just about anything but I'd bet on a PA28.
You might want to look out for much cheaper training rates on diesel versions as they are becoming more popular and could amount to a £2k saving on an average 50 hour training requirement.
Some schools offer up to £400 worth of 'student pack' including all books, ground school and ground exams, headset etc for a 1/3 deposit. I do understand this makes some people nervous but if you pay by credit card you have protection. Some schools have escrow accounts which also offer buyer protection.
Everyone interacts differently so asking others what an instructor is like has limited use. Personality is everything, flying should be fun but learning can be a bit stressful so you need a nice instructor to fly with, but one who will push you when you need it. I'd go with your gut feeling at first and if you don't really get on then change instructor early. The best people to speak to would be the operations staff.
Hope this helps, every success with your PPL!