Feels like you’re over-leveraging yourself especially if you’re already planning of having a lodger.

I'm not sure that's the case, as long as there is no massive short-term cash need before

The mortgage repayment is likely to be £1,754 (30 years, 5-year fix, 3.29% @ 85% LTV).

Even allowing for a voluntary 11% pension contribution, £75k gets you £3,872 monthly after tax... so over £2k of post-mortgage income before bills and any income from a lodger.

Assuming the house is in a safe area and not in immediate need of repair, and there is enough cash left over to actually move in, I think it's fine. Sure, it's not going to be quite the lifestyle they're used to, but it's perfectly doable.