Boundary Search x miles from original Boundary

I can across this article and found it very useful:

I am trying to achieve something a little different and I thought you might be able to advise.

On Rightmove.co.uk you can perform a radius search:
https://www.rightmove.co.uk/property-for-sale/map.html?includeSSTC=false&keywords=&sortType=2&viewType=MAP&channel=BUY&index=0&radius=3.0&locationIdentifier=REGION^884

3 miles on this search is not correct however.

I am not looking for a Geo Polygon query within a shape in effect or a geo distance query from a point:

I effectively need to create a search from a similar polygon at scale:

In other words I have a geolocation boundary, I need to scale by 3 miles and find all locations within the bounds.

I was wondering if you had any idea how i could perhaps achieve this?

I am theory looking to build a property site that performs searches across the whole of the UK and your help and expertise would be most appreciated.