Since you are Irish as well as British, Brexit won't change anything for you.
Cesvaine? Lovely place, I still have photographs of the ruins and Teici Natural Reserve. About work opportunities, did you consider teaching English?
>>>>I’m starting to think a move is much tougher than I had hoped though.
Not really. I have done my fair share of moving and have to say that living all your life in the same country is a bit overrated.
Having said so I have to admit that it should be a bit more difficult if you have small children. I think they will love it there. Apart from cold weather it's a lovely country. The immigration officers have always been really nice and helpful. I am not sure how it is done now but back then you go to the immigration office in person which surprisingly makes things much, much easier than sending out 1 kg of papers to HO and receiving a copy-pasted reply by post. You can ask questions and they will help you out, they are very helpful and nice people. I had moved to Latvia twice (I lived in the US in between) and had nothing but good experience dealing with the immigration authorities in Latvia. You do know that you don't really have to get a residence permit if you want to stay in Latvia or any other EU country for that matter (not sure if this rule is followed in Spain though). You can but don't have to. It will be easier though if you do for the purpose of buying a property and getting a loan (I guess) or a job...
>>>I have been considering renting my home out in Bournemouth. Would having this income be considered as self sufficient or would I have to sell my property (not what I would want to do) Is there a minimum amount to be considered self sufficient?
House at seaside? You will be laughing. A house or a flat, it can't be less than 500 a month. It's enough. I asked a question on the plmp.gov forum (yes, they have a forum where immigration officers will answer your questions. It's in Latvian though) and I will let you know when they answer it.
>>>Of course I would have to adapt to the winters but the call to be near family and escape the pressures of ‘developed’ UK is just too strong.
I can relate to that.
>>>The appeal of chopping wood, fishing, and chickens in the back garden really spurs me on.
You can cut your own Cristmas tree you know.
Forest covers over 40% of the territory. The nature is stunning, you gonna love it.
>>>>The thought of my son being in a class of 12 and learning to drive at the school instead of being ripped off in the uk. Not to mention beer for 90p and real food. Too many things appeal.
Proper rip off, indeed. I had Golf Mark III and paid 30 quid a year for car insurance. Yes, I am not kidding. And yes, driving could be a bit tricky in winter, but you will manage. And fog (though looks beautiful when fog forms sort of layers that float above the ground, still significantly diminishes visibility) - you will get it often.
If you buy property, buy a house (getting a good chunk of land by a river is not a problem). You can't really own a river in Latvia, but you can own a bank. Houses are often heated by solid fuel (wood in particular) not because Latvians can't master heating with gas. They like it this way. Cheap and smells really good.
>>>Learning Latvian will be a challenge but it doesn’t scare me. Can it be any harder that Engineering Mathematics?
Slowly but surely.
It's different.
Language is a time-dependent skill, the more time you spend with it the better you get. Since you mentioned math, may I give you a piece of advice? Don't follow your natural instinct of approaching this task analytically (I mean studying grammar). You will end up with experiencing a typical case of paralysis through analysis when you know all the rules but can't speak (I learned it hard way studying English). Start from listening and speaking and leave grammar, writing and reading for later stages.
>>>>Moving countries is a massive step but how do you know if or when it’s right to step over that line?! How do others do it without being in a position to burn money for fun?
I changed country of living six times (I don't mean traveling, I mean moving to another country).
It's really simple (not easy sometimes but simple): get prepared, do your math and try to embrace the inner adventurer. And have a bit of faith in yourself.
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Slowly but surely. 