We went from 80K to 138K and our payments went up by £350 ish. But that was going back up to a 25 year term. Nationwide (and other lenders as well I guess) have a calculator on their website that lets you work it out.
if you go back up to a 25 year term it doesn't mean you can't overpay it and clear it sooner. So in the short term you could have a bit lower payments and then increase them later to pay it off quickly.
If you need more space it makes sense to do it now so you benefit from it longest and (hopefully) also you buy before prices rise any further.
Also remember that the recent interest rate rises have not really filtered down (takes about 12/18 months for their full effect to be felt) and that also interest rates will most likely being going up again.
Extending tends to be a similar price to buying up from what I could work out from months of fcking with architects, planners and builders and then giving up and deciding we would all sleep in one room