I am looking to better fill up the arch space, give it a more substantial look. I think it looks a little perched on its current wheels. And to fit a tyre suited for mild off road use. But I want to keep the classic look hence the white steel wheel... yes a budget but that’s just not to spend masses, I’ll have to be guided a bit buy what it’ll cost
Not really sure what you mean by fill up the arches? In which dimension?
Are you actually going to go off road? If the tyre is too big, it'll rub. Big tyres can also blunt acceleration and run the risk of breaking diffs/axles.
235/85R16 was the mainstay standard size for a Defender. It is a good size and works well. I believe 265/75R16 was also used in some markets. About the same height, but wider. You might need a wider rim.
7.50 x 16 is an older size but also standard fitment. About the same height as a 235/85, but narrower.
In all cases the standard steel rims will sit the wheels very inboard. Wheel spacers or wider off set rims may well look better and allow less rubbing and more steering lock.
There are lots of variations of standard steel wheel. Early ones require tubes, so opt for latter tubeless ones. I think they are all 5.5" wide, so pretty narrow, you won't be able to fit a wide tyre to them. The wheels on yours at the moment are Range Rover Roystyle rims. Not sure if they were fitted to some county spec models or not, but you do see them on some era of 90. They are 6" wide, but a different offset.
Land Rover 130 rims (or Wolf rims) have a modular look with holes around the edge, these are a slightly better offset and wider.
There are aftermarket Land Rover style rims these days too, some as wide a 8" allowing an even bigger choice of tyres to be fitted.
If you want to go 'bigger' a 255/85R16 is quite a nice size and tall. About as big as you can fit on a stock 90, but it may still rub off road. Check a tyre makers site for rim specs for tyres.
285/75R16 is another option, but is likely to stick outside the arches, which will be illegal. This is much wider and will require a wider rim. Lot and lots of other choices, really depends on your actual goals, what compromises you are prepared for and how much you want to spend. A new set of rims and tyres could easily set you back £1600. But there are other options that are cheaper.