Try this link https://www.themelton.co.uk/mortgages/guide-to-self-build/#2
They mention what you are trying to do ,
In areas of high house prices, it may be worthwhile purchasing a poor quality house and then demolishing it to create a building plot. For example, some areas have poorly built bungalows on fairly large plots surrounded by larger houses. However it’s always worth speaking to the local planning office to gauge their attitude to this type of development.
But also mention,
To qualify for a self build mortgage, your project must usually meet the following criteria:
Outline Planning Permission must have been obtained before your application can be considered and before any initial funds can be released.
I found them very helpful to talk to but they will only deal with a broker when it comes to applying for a mortgage, they are one of a small pool who will lend on just outline PP