You seem to be misunderstanding how equity splits work and what happens with the mortgage.
The house value is £280k, the outstanding mortgage is £45k. That means you are playing with £235k of equity. That's the bit you own and the only bit that comes into the split.
If you were to go down a 60/40 route in your favour. You get £141k of equity and he gets £94,000. The remaining mortgage doesn't come into it.
You then have options. If you want to keep the house, you take over the remaining £45k mortgage, and you eventually buy the rest of it. Or, you sell, walk away with your £141k, and buy somewhere new (either outright or by adding a mortgage).
If you only give him £60k. That's 25.5%, giving you 74.5% of the assets. You'd have £175,000 of equity.
If all you're splitting is the house, a judge is unlikely to look on this as a fair or equitable arrangement. A PP mentioned they'd got a similar split, but that's because they'd offset the pension. So the overall asset split was back to 60/40. If you could prove he had a bigger pension or tons of savings or went after the business, you might be able to offset similarly. But if all that's on the table is the house, I'd be surprised if he went for this, and it would be unlikely in court.
Your confusion seems to be around the £45k mortgage, but this is irrelevant. You don't get to deduct that from your share and say "look it's 60/40 now". You can choose to take on that mortgage or not, but what you're haggling over is the existing equity, not the future equity.
If in an alternative universe you both kept paying the mortgage till it was cleared, then he would be owed more than he is now. Because you'd be looking at a share of the full value not the current equity. (He'd need £112k to get 40% - more if house prices rose).
You can't factor your potential choice to pay the outstanding mortgage into the equity split. That's not how it works.