I have never found a british insurer that will insure my car abroad for more than 90 days
If you want more cover than just 3rd party only, then yes that's a problem.
But why not just get a 3rd party only policy from any insurer in the UK? Did you know that any policy bought in the EU must cover you to the minimum legally required level in any other EU country?
i.e. if you buy any policy in the UK then you are covered to 3rd party level in Poland, whether they know you are leaving the UK or not.
Clarabelle has pretty much covered everything in her post. I registered a UK motorbike in Poland and it cost 550 zloty, I would expect a car to cost a similar amount but as has been stated you can't register a RHD car here.
Regarding the tax, I had a UK van in Poland for over a year. The problem is that in the UK you either have to declare a SORN (statutory off road notice) to say your car is off the road or you have to buy your tax disc. If you do neither then you automatically get issued a fine.
The DVLA say the vehicle must be taxed even if abroad - being abroad doesn't count as off road. However no-one outside the UK cares if you are displaying your tax disc or not so I can't see how they could catch you if you did declare a SORN.
My tax disc expired whilst in Poland. I bought a new one online. I drove around for many months in Poland without any tax disc and never had any problems. Of course as soon as I came off the ferry I got pulled over by the Police in the UK. I explained that I had been abroad whilst it expired, my tax is paid and the disc is at home (in the UK). They were happy with that (they checked on their computer), although reminded me that the offence is "failing to display a valid tax disc", rather than not having tax.
Regarding Polish MOTs, as has been mentioned this won't apply to you because you will need a British one - but yes they are indeed somewhat less arduous than in the UK. Last year my wife asked her cousin to get an MOT for her. An hour later he simply turned up with the certificate (car had been in our garage the whole time). This year when my wife asked him, the cousin rang an hour later and said "this time they want to see the car". So I guess standards are rising....
Having said that the MOT for my bike this March lasted the whole of 10 seconds whilst they checked that it had 2 wheels and headlights....