Should I sell my buy-to-let flat or keep it?

theguardian.com:

Q In 2006 I bought a flat on the south coast. Looking back I had no clue what I was doing and got caught up in an unrealistic bubble that burst soon after. I live in rented housing in the south and couldn’t really afford to buy anything. But I thought I would get a buy-to-let mortgage and that holiday letting would pay the mortgage and that at some point I would own it as I would otherwise never be able to buy in the area I live as my work has always been unstable freelance.

I was refused a buy-to-let mortgage (they said there was no guarantee it would let for enough weeks but I knew it would because of the seaside location) so I asked for a domestic mortgage, planning to live there within a couple of years and thinking it was no big deal. But my circumstances changed and I have not moved.

Now 15 years later it has been a successful let, covering the interest-only payments and the costs of managing it. But it has hardly reduced the bulk of the mortgage which I likely paid well over the odds for.

My question is should I keep it in the hope that there will be more money in it in 10 years so I have more for retirement? My other main worry is that the flat has been registered as a business and I pay tax on it but the mortgage company think I live there. I worry if I do anything I will rock the boat and flag up the issue, I’ve read that the mortgage company can demand the whole payment (I owe £190,000). If I sell does my mortgage company know anything other than I am selling? If I keep it and try to get a new mortgage with another lender, will it be flagged I live elsewhere?

If I sell I won’t be buying anything else I will subside my low income and have a better life for a few years. It won’t be enough into older age but at least