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We've just moved almost three years into a 5 year fix in a similar scenario to the one you outline. We've now got an additional 2 year fixed for the extra amount from the same lender, which is at a better rate than the 5. Plan is to go onto the variable rate for the ~3 months between the 5 and 2 year terms ending, and then consolidate them both.
If we'd moved a few months later I think it would have made more sense to pay the ERC and get a better rate, but we moved when we did so ¯_(ツ)_/¯. Only other thing is they'd only give us the two year fix for a 35 year term, so the 5 year is now also on that rather than the 31 years it had left, but I don't really mind that. I don't think the lender charged any fees, but our mortgage advisor who dealt with it all charged a bit for the admin.
These mortgages are portable. The only* fee would be an arrangement fee (£1k) for an additional mortgage to cover any difference.
In case it's useful let's take an e.g.
Current House = 400k
Mortgage = 300k
New House = 500k
That means we'd need an extra 100k to buy New House.
Potential issues I see:
*we think - hence me asking in case people have experienced any surprise fees.