@d2d4j - I just rechecked UKServers' quoted mail storage limit is per-domain, and all the mailboxes allocations must add up to less than that total.
So for example, I have 250MB for a main mailbox, 40MB and 40MB for two others, and this adds up to less than 400MB total, so the rest is currently unallocated (because I can't add up). I could therefore increase the size of the first mailbox for example, which currently has 2MB of data sitting in it, because I haven't been bothered to clean it out.
I could set it to 'age out' the data, which is probably what I have already done, can't remember. [Usually my customers’ (when I was a consultant) mailboxes were always nearly empty as they were being continually emptied out every few minutes to hours by some client machine.]
When you want more than 400MB storage, I would ask UKServers’ staff for their recommendations, just to check. Buying an additional block of 10 mailboxes gives you an immediate extra 400MB total for that extra block.
If you need more than 400MB per individual mailbox, then I would have to ask them about that. If someone has a requirement for a lot of storage in an individual single mailbox (for example, if you are never cleaning one out) then this is a red flag. In fact, I will ask them just so that I know.