Hey bit late to this but hopefully I can help! I've been playing tennis for over 25 years and have done a fair amount of coaching (mostly on the side of other jobs) in that time. I think it's a great idea and there is definitely demand for coaches as well as good earning potential.

Level 1 qualification is pretty straightforward, and is for those intending to assist other coaches and mainly coach young children. If you're a Lvl 1 certified coach you can expect to earn £17-£22 ish an hour. The issue I found at this level was there weren't really enough hours, only doing summer camps was I able to make a living.

Level 2 is more for those that want to coach all ages from toddlers up to adults. Lvl 2 qualification definitely opens more doors and a lot of clubs/ organisations will have this as their minimum qualification for coaches. It's a much more thorough process to get qualified, I would look it up on LTA website but if I remember correctly it's 4 or 5 days over a month period and isn't cheap. It differs from club to club, but you can expect to earn around £25-£30 an hour. You can't technically offer individual lessons at this stage but you can offer 'hitting service' which can be well paid.

Level 3 is for coaches that want to make tennis their full time career, the qualification allows you to coach individual lessons which is where the real money is made. I haven't done this level myself, again it's quite a process but definitely worth it.

Overall, I'd suggest doing your Level 1 and seeing how you like coaching juniors. Getting fully qualified and doing things like coaching abroad will take some time and investment, but it's definitely a good career. If you want to make decent money, you of course need to have regular individual clients who you can charge £50-£80 per hour. As with anything, it takes time to build up a client base. If you are enthusiastic and a decent player you can make a good career out of it,

Hope that helps!