So presuming that I do move to Spain before retirement, what are my job prospects?

Well, I've got to look at what I've done in the past for work and can I do it in Spain. So lets look at my history.

I left school and became a Drivers Mate. I don't think there's a call for that type of job these days, not even in England.
I then worked in a woolen mill sorting rags for a few months. Not much to go on there.
I joined the army for nearly seven years. I'm now 48 and Spain has it's own army, so no need for an aging ex-squaddie.
I then worked as a groundworker. Well unless you're Spanish, you wont get in with a big company employing groundworkers.
Then I became an HGV driver. My licence ran out at the age of 45. These days you need a digital tacograph, driver CPC and because I'm over 45, if I did renew my licence, I'd need a medical every 5 years. On top of that, I'm not Spanish.
After giving up driving at about the age of 30, I became a warehouse foreman for Proton and Lotus Cars until redundancy. Again, I'm not Spanish.
Whilst I was at Proton/Lotus, I joined the Navy part time and made full time for a stint when I was sent to Iraq. I don't think the Spanish Armada have any need for an aging British sailor.
I then got into the building trade, self employed. I was building conservatories, extensions, general building work and then gabion walls all over the UK. The only real way to do that in Spain is to be self employed, (autonimo). There's already lots of people doing that in Spain with a lot more experience and proffessionalism than me.
Then it was back to warehousing and production as a manager, again I'm not Spanish.
In between and along side these jobs, I've also done bar work. there is plenty of that, but mainly seasonal.

So what's left. The only thing that is left for me is a change of career as none of my experience is any good in reality.

I always though about becoming a teacher when I was younger and to get to the point, that's my chosen route and I'm pretty sure it's a move that I'd make even if I wanted to stay in the UK. I did a TEFL course back in 2014, but that on it's own is a waste of time, look at the competition, to get ahead, you need a degree to go with it, which I'm doing (in a relevant subject that also helps). even then there's still competition. To build on that and make yourself stand out, a PGCE on top would be the minimum requirement in my opinion. Whatever route, it's a slow process but I'm sure it will be worth it.

There is plenty of prospects out there, for those with experience and a niche market who can go self employed. For the rest, the market is limited. Seasonal bar work, teaching (even that can be seasonal), Chefs are always required along with people with experience in real estate.

I may be wrong, but for me paid online work is a no go. Try getting past the scammers promising to make you into a millionaire within a matter of weeks. Pay per click jobs are not worth the time, the same as surveys, you may earn a tenner every six months and have your inbox full of spam.

Don't dismay though, for a lot of people out there your skills are transferable, just not mine.