It is a simple math problem, but the problem is you don't know all the numbers, so you have to make your best guess. Interest rates are not going down unless the apocalypse arrives, then it won't matter! So, the important questions are:
What is your current variable rate?
What is your maximum increase, and how often can it adjust?
How much "shock" can you comfortably absorb in your variable rate before it becomes painful?
What is the fixed rate you can get today, and at what cost?
How long do you have to pay off your mortgage?
How long do you expect to live in your current home?
If you know those answers, you are a long way to knowing what SHOULD save you the most/cost you the least.
HERE is one example of the types of pre-made calculators you can use, but the web is full of them. You might need one specific to the UK market, as your "fixed" rates do not generally seem to go out to the full payoff period of your mortgage,so you have to ladder "fixed" mortgages over time.....sweet deal for the banks!! Rip off Britain, indeed!
I'm not sure how useful "back of the envelope" calculations are, but as an example, if your mortgage rate can adjust up .25% every year and the 5 yr fixed rate is 1% higher than your variable rate, and you plan to move within the next four years, then you have no worries!
However, if your rate is REALLY variable (as in tied to a market rate, for example LIBOR +, with NO upper limits or speed of change), then you are what would be termed "naked" and at the mercy of the market. Rates will rise, so your ONLY question is can you absorb rapidly rising rates without losing your home. That sounds dramatic, but when it boils down to brass tacks, that is the real question....not can I save a dollar.
So, without more info, it is impossible to know which is best for you, but if the only question is whether interest rates are going to rise or fall over the long term, the answer is rise.
HERE is an article that might have some info of interest.
Good luck!