Look for a better interest rate on your mortgage. I’m on a fixed rate two year deal with an interest rate of less than 2.5%. You could save a bit there.
Leisure is quite a big expense for you. You could look at reducing that, and setting a ceiling budget of perhaps £300 per month.
Drop your food bills. We made a big dent in ours by having two vegetarian or vegan nights a week and one night eating something really cheap, like jacket potato and beans or dhal and rice.
Sell one of your cars. Even if it’s the old banger, someone will want it and you then won’t have the running costs of petrol, MOT, servicing, etc, going forward.
Check all your insurance and energy bills in case there’s a cheaper deal somewhere.
Finally, find some way to earn a bit extra while you’re starting your business. My husband and I made a commitment to raise an extra £500 a month for the last six months to help me cover startup expenses. We ebayed stuff, sold ideas and stories, I got interviewed in Take A Break magazine - just whatever we could think of. It funded my website, some advertising costs, a pro photoshoot for images - all the stuff that would have been difficult otherwise.
Happy to chat further as I’ve recently started my own business so may be able to answer some of your questions as you go through the same thing,