From your figures Schmaltzing, I would continue the endowment, but if your finances allow convert the whole of the mortgage to repayment. In an ideal world, then you have all outcomes covered and get to keep your home.
I expect that if you cash in your endowment now you would be hit with penalties.
I think its great that your mortgage is so low compared with the value of your home, it could be a hell of a lot worse.
As a mate I would say, convert all of your mortgage to repayment and keep your endowment. If you are near retirement age or on a very restricted income it may not be possible, and then I would try to be aware that it would be good to save in cash as much as possible to meet any potential shortfall of the endowment. There might not be a shortfall, keep your eye on the statements.
If you sell up anytime now between the 2021 maturity, you can pay off that mortgage with the proceeds of your house, continue the policy that matures in 2021.
I don't know how old you are or what your income is. These would both be factors that could make a big difference. Unfortunately the younger and richer you are the better, but nobody needs to be told that.
What I would do personally if I was on a restricted income or approaching retirement age would be to ensure the roof over my head and clear all debts.
Sorry if my advice seems really general, its just what I would do, to protect my home and my family.