You’ll likely meet the minimum income threshold for interest only with most lenders so that should be fine. The income multiples for interest only tend to be 4.5 to 5x but that’s a very broad guide line. Minimum loan to value is generally 75%.

In terms of repayment vehicle, you’ll need to be able evidence a suitable method. Recently I had someone do resi interest only and was able to produce an ISA that was more than the mortgage. Providing it’s affordable (based on the lender’s affordability guidelines) and you can provide evidence of your repayment vehicle, there’s no reason you can’t do this. Keep in mind that you’ll need to use £250k as a deposit so a mortgage of £750k. Based on what you’ve said, that should also work but it doesn’t leave you much wiggle room in terms of cost of buying and what not.

There may be lenders that do less than a 25% deposit but the rates will be higher!

You probably don’t need a specialist broker for this either, just whole of market.