How long do you expect to be in the house?do you expect to keep it for the full term and own it outright in 21 yrs?
In the States, the average person moves every 7 years, but they have 30 yr mortgages that barely pay off any of the actual mortgage and are heavily interest payments...because they are cheaper and homes have historically gone up in value. When they don't go up, you can end up underwater, owing more than the value of the home...or simply losing all equity value when they move!
How do you sleep at night over money? Do you worry about it? If so, take the longest FIXED RATE you can get, especially if it is only about £1.50 extra per day. How much would you pay for a good night's sleep?!
There are too many unanswered questions (what's your credit profile look like?! That's a biggie that is personal, so I'm certainly not asking!) to actually give you the "best" answer for you, but you should go through at least three companies to get their best deal and compare not only their rates, but their guidance/information.
We are living through a historically low interest rate period, even with recent increases, so "locking in" a fixed rate for an extended period provides you the best protection against any inflation spikes, especially with the looming Brexit dysfunction!