To add my 2p worth – no, the boot isn't anything to write home about with all back seats in their normal position. But with 1,2 or 3 seats folded forward or, even better, removed, there is tonnes of space. Yes a lot of it is up and down rather than sheer area but unless you just want to place bags of shopping side by side that's really useful for big/ awkward items (like bikes or getting changed in the back). The height and a low footwell means the back is also pretty comfy to sit in for taller people/adults.
IMO there are plenty of big estates with bigger boots when the seats are up but the folding options and removal make Roomsters pretty versatile. They aren't huge externally or internally - Berlingos, Doblos and their ilk are way bigger (especially the new versions) but the few I've used are by comparison terrible to drive. But for my needs the roomster's “usable space” with seats out is as good or better than all but the very biggest estates, and those are likely to be way more expensive to buy and run etc... If it's in your budget I'd go and try a roomster and play around with the seats (i.e. if you don't need 3 in the back all the time see how that works), see how it drives before writing them off.
Just seen budget of 7-9k, should easily be within budget but you might fancy something "nicer" seeing as you have the cash.