A few more things you might be interested in while they're still going cheap in the post-holiday sales:
Recent Hugo Award-winner Liu Cixin's
The Dark Forest, 2nd in his Three-Body Trilogy of which the 1st book was the award-winner, is just £0.99 from Head of Zeus (published by Tor in North America).
Tim Powers'
Declare is just £1.79 from Corvus, and I very highly recommend this as an awesome mashup of John le Carré and biblically-referential Lovecraftian sensibilities for a brilliant Cold War secret history horror spy thriller. I may be biased because I absolutely loved it when I read it, but since IIRC you enjoyed Charles Stross'
The Atrocity Archives in his Laundry Files series, I think there's a pretty good chance you might like this one, even though it's somewhat different and darker in tone. Stross mentions it in the essay in the back of TAA about his influences, which is how I came to know about it. Fair warning: it is a very densely plotted and demanding sort of read (Powers apparently based it upon some sort of rule that he couldn't alter any actual RL historical occurrences when writing it, only come up with interstitial additions and hidden motivation explanations for it all), but well worth it if you're interested, IMHO.
Head of Zeus also has on sale some C. J. Box mystery novels for £0.99. I haven't read these, but they're award-winners, and we did get the
1st-in-series as a UK freebie a while ago, and you may want to have a look at
their other listings, which include some historical fiction at the £0.99 mark, including one by Sharon Kay Penman (apparently a very respected and popular historical writer whom I've never tried, but IIRC is a favourite of several MR members).
ETA: If you keep paging through to the £3-4 mark, Head of Zeus also have a bunch of Brother Cadfael mysteries if you don't already have the lot, and some of Colleen McCullough's historicals, including entries in her Masters of Rome series, which were kind of flawed, but which I enjoyed quite a bit, and some Dana Stabenow, who's been offering the 1st in her Kate Shugak series as a long-term introductory freebie for some years now.