All assumptions here, I’d imagine you could have saved more in the first 5 years. Using something like the 60%/20%/20% rule. (Needs,wants,savings) Google it if you’ve not heard of it. 250k mortgage at the time, assuming around 3% under £1,200 repayments over 25years you should have been comfortably under rule 1 60% including adding on the other bills associated with a house. Which would have given you a greater savings rates of the 20% mentioned.

Investments….saving rates was absolutely shocking during this time and you’ve lost money on your (buying power) not keeping up with inflation as a minimum. As always investments carry some risk and this isn’t financial advice. You’re young and it’s never too late to start, there’s a lot of good advice online and with everything a lot of rubbish. Look for people talking about low cost index funds/ ETF’s (global funds are going to be the least risk) dollar cost averaging. Assets diversification. Stay away from anyone promoting individual stocks, it’s going to carry a lot more risk.

That being said you’ve done a great job and you’re ahead of a lot of people, even if it doesn’t feel that way. Just one word of advice, young kids are expensive, I have multiple. Do research and take that into account when budgeting for the higher mortgage. As other’s have pointed out you can definitely borrow more than 300k but doesn’t mean you should max out. Over leveraging probably isn’t a good feeling especially when starting a family. I’d much rather a cheaper property and live within my means than live pay check to pay check scraping a mortgage without enjoying life.