Wow - That sounds like you are very financially secure! Great update.
It sounds like you’re well able to cope with additional mortgage without excessive risk and you like the house you’re thinking of buying - all good. I don’t think you need my advice about house buying!!
You have however asked if it’s a good investment financially, so let me run some quick figures…(making some broad assumptions for comparison):
Additional £200K mortgage over 25 years at 5% interest.
Total repayment approx £350K. Monthly payments £1,169.
Staying put and saving £1169 monthly for 25 years at 5% interest = £703K
Investing £1,169 monthly in an index fund such as S&P 500 (average returns annually over 25 years about 9%) returns £1.33 million over 25 years on past performance.
Additional returns on the new house purchase - unknown, but past 25 years has been about 3 or 4 times its initial value depending on location… so the extra £200K value might be worth about £600K to £800K.
Of course past performance no guarantee of future returns, but what else can you use to make a comparison?
Extend the mortgage over 30 years and the gap in likely returns would be even greater.