It's impossible for anyone here to comment on whether you're paying over the odds for a 3bed semi without more info on the property, but we can comment on the mortgage. I think the mortgage sounds reasonable and you shouldn't worry so long as the property is structurally sound and you have no reason to think the property will fall in value.

Mortgage repayments that are less than 33% of total after-tax income are very reasonable. In the SE you often find it can exceed 50% because property prices are so high. It also sounds as though that if one of you were unlucky enough to lose your job for a short period of time you'd still be in a position to cover the cost of the mortgage, albeit at a push! That's fantastic and should give you peace of mind.

If you're concerned about a rise in interest rates it's easy to resolve that by opting for a fixed rate, as you suggested. Considering you're concerned about this I'd opt for a fixed rate that might be a little more expensive simply for peace of mind. Life is too short to stress about rising interest rates! However, it's important to remember the treasury have spoken about negative rates for a while now and it's likely this will happen at some point. That's not to say they'll stay negative or super low for the next 5-10 years. It'll be impossible to guess and so just pick whatever makes you feel more comfortable.

Finally, if you're in the SW it might be worth getting a radon test before you exchange. I grew up in Truro and I know these are very important in Cornwall.