Funnily enough, I noticed Captain Earth was down to £21.50 for the Blu-ray parts the other day and thought that was a decent price. I think that'd be about the same as the cheapest half series new release I've bought in the last three months or so and seems to be fairly standard for MVM releases (sometimes they don't drop below £25). To be fair, with delays and such I haven't bought too much in the last few months.
Looking ahead, the picture looks similar. Maybe there are a few more £25 or above releases and not so many below, although that could change (plus, as Mangaranga mentioned, there are a fair few collector's releases and such).
Index/Railgun are both £36 at the moment but since they're 24 episodes that's equivalent to two £18 sets (Index was also slightly lower earlier). That doesn't seem so bad.
The other online shops often have slightly better deals than Amazon. I find that Amazon tends to have better prices for pre-orders or new releases and then they tend to rise once they've been out for a while. Anime Limited releases seem to vary a bit more and be more likely to drop from time to time, while MVM, Manga and so on tend to stay about the same or increase significantly (at least on Amazon). I think Movie releases in general also tend to vary more in price and are more likely to come down after release.
I tend to consider about £25 standard for 11-13 episode releases these days. Still, with prices like that there's less incentive to wait and pick up the UK release since it may only barely be cheaper than importing from the USA (plus they're often much the same release anyway).