I will pay off by selling and moving. I can't have a mortgage this big till retirement. Too many other demands on my income.
This could cause problems because we don't know what is going to happen to house prices. Over the past few years, in most of the UK, they have barely risen at all, so unless you are in London or the South East this is not a good strategy. I've played with some figures, looking at the cost of an average 2 bedroom and a 3 bedroom local to where I live. The average cost of a small modern starter home 2 bedroom is £140,000 and a similar 3 bedroom is £165,000. For calculation purposes I've presumed a 100% mortgage.
To borrow £165,000 over 25 years at 5% interest would cost £965 per month in a repayment mortgage, or £688 interest only. The total paid in repayment would be £289,500 and you would own the house. The total paid in interest only would be £206,400 and you would still not own the house. If we allowed a conservative 1% growth in house prices, your house would be worth £209,506, so if you sold it and paid off the mortgage you would only come out of the deal with £44,506, which is not enough to buy a house and will be worth even less in 25 years.
You need to sit down and look at the various options - what sort of house would you want to buy when you sell up versus what you are looking to buy now and do a similar exercise. You also need to look at house price growth in your area.