I thought I would offer some information on my original post. I took the plunge and attended a 1:1 bespoke training day with AirlinePrep. I have to say it was very useful and they helped me prepare for my recent cadet pilot assessment.
The instructors are current First Officers with two major UK airlines and the training is carried out at a UK Airline's Training Centre where they have the use of some conference facilities. Learning in this environment and seeing Pilots and Cabin Crew go about their training really confirmed to me that I want this career.
On the day, the Instructors took me through a Group Exercise, assessed my numerical reasoning skills (and gave me some hints, tips and training on areas I was unsure about), mechanical reasoning skills (gears, pulleys, simple electrical circuits) and conducted a one hour long interview. The interview was very challenging and they really helped me with the competency based style questions such as "tell me about a time you have disagreed with someone more senior" or "tell me about a time when you have lead a team". They helped me add Pilot specific skills to my answers and structure my them in such a way that I was conveying all the relevant information to the Interviewer. Many airlines have a Pilot skills list, so I found it valuable knowing what these core skills are.
The Instructors have also kept in contact after the course and helped me with any queries I still had. I would certainly recommend contacting them if you want to give yourself the strongest chances of succeeding.