I'm an architect specialising in interior spaces and I would point out a few things. Double height spaces usually have acoustic challenges and a mezzanine can help mitigate that somewhat. So if you plan on having a TV or for the seating area to be a heavy social traffic area you may find yourself having issues with sound if you remove the mezzanine.
Also, whilst large double height spaces look really impressive they can actually feel quite uncomfortable psychologically, especially when you are seated lower. What works in a church or chapel (vast spaces instill a natural silence) doesn't always translate well in a home setting iyswim. Double height spaces also tend to dwarf furniture so I'd take in consideration added costs with this as things will often have to be customised or sourced at higher cost due to size.
Without seeing plans/photos I couldn't say one way or the other, if the ceiling is especially low under the mezzanine (as in you could paint the ceiling without the use of a ladder) then I could understand the inclination. It may also be a case of proportions, eg. if the mezzanine feels oversized in comparison to the rest of the room I can imagine it can feel somewhat oppressive.
For an honest appraisal of what is possible technically, seek out an engineer instead. I say this as an architect myself, we're usually trained to be designers of space with a focus on design and whilst we understand the theory and can do the calculations it's usually not part of our everyday practice. So we can easily say something can be done (and honestly almost anything can be done as long as you've got the funds, so it's an easy answer to give) and give a rough estimate of costs but an engineer will do a more thorough job and avoid situations where you're left with a stray constructive beam that needs an engineering and/or design solution, adding more unexpected costs. In new construction, things like mezzanines actually often are the result of an engineering solution dressed up as design rather than an intentional design feature, especially since they come with added costs vs. standard construction solutions.
Another thing to keep in mind (full disclosure I'm not in the UK, so may not be the case where you are), if the value of the house (and therefore your mortgage, etc.) is based on price/m2, physically removing floor space will impact that value and theoretically lower it on paper.