Well the really good bit first:
You have the Lian Li PC7+ very well priced, but if you could confirm it was 120mm x 120mm and add £3 not £40 like a competitor to the price you would probably see a few more sales.
Onto the bits that I really think you need:
Stock the Lian Li PC7+ side window, and label it as compatable with the PC60 and 7 as the amount of people asking for it and them having to go for a competitor is silly.
You should stock more of the low end Silverstone Lascala cases, having the high end cases is all well and good but most people on these forums at least want the ones below £100 as there HTPC systems are not there main rigs.
You could do with all the new Lian Li cases, bar maybe the G7 as you have the PC60 and PC7+ and its the same case with a different front, may conflict with those two cases.
PC60 should be cheaper, although I know it costs more to get in; compared to the PC7+ in a value for money point of view it is slightly over priced.
You could get rid of the Asus Vento as its a horrible case.
One idea is doing case bundles, with a set of fans and a fan controller at a slightly discounted price, but compatable and everything as there seems to be a little call for that as far as I can tell. Maybe a Thermaltake Soprano and an Akasa Eclipse 62 bundle would be a good idea, with a few ££ off for getting the bundle.
There are a couple of decent OEM cases, but I do not know there proper names only the ones branded from competitors, but they would be a good addition to your OcUK value range.
Your Antec Super Lanboy is priced at an amazing level, and comparing it to your OcUK Value range it offers a huge leap in what you get for your money.
I have more but its not forum appropriate due to case names. If you can get Windy cases in at some point it would be a good move, although I know that they are not ok in the EU yet.