Advice on moving internal 1st floor wall

Hi, I'm a homeowner in the UK and have two adjacent bedrooms on first floor, one very small. I'd like to knock down the existing wall and get a new stud wall put in to make the rooms similar sized but I don't know where to start. Just to be clear, I don't intend to do any of the work myself (except maybe painting!).

I would appreciate any advice on what to ask for from a structural engineer/builder to make sure that, if this is a structural wall, it is properly removed and supported. Thanks!