I saw this question on here, so I thought I'd share what I found out on this subject.
The first thing is that interpreting the data is quite tricky. Many sites such as Reed have too small sample sizes. Payscale is good, but even then, recognise that this is *salary*. As such, it excludes the heights of what personal trainers who can stick through it and start their own business can earn.So, Payscale says that for personal trainers in the UK, the average (mean) salary is £19,323 a year.London as the capital is a bit higher.
If anyone has any more data points or corrections for me, just reach out on my website. The aim is for it to remain accurate and useful.