They're miniatures, measuring 32mm wide by 42mm long. Being miniatures, this makes them quite difficult to date.
Normally, I'd show you three picture cards, but this time I'm only showing one: the Queen of Spades. This is to establish that it's unturned (that is, the spade pip is top-right rather than the top-left it is on modern cards).
The Ace of Spades is just a single spade symbol, rather than the elaborate version featured on larger packs. It doesn't even nod in the direction of elaborateness. British Aces of Spades are normally fancy, as a hold-over from the days when the manufacturers had to use an intricate ace ("Old Frizzle") to show they'd paid tax. Such cards were all the same size, though. I don't know if small packs were subject to the same tax laws as larger ones; they might not have been, but then they may have been illegal.
As you can see from the 2 of Hearts, this is not a well-made pack. It's been printed onto a sheet which has then been guillotined. Because the printing wasn't central, the guillotine took the top off the 2H, 4H and 5C, and came perilously close to doing the same to several other cards, too. The 4D is missing the very tips of the diamonds along one long edge. Curiously, though, none of the other cards show the snipped-off parts on them. None exhibit the rest of the 2H's pips on them, for example.
Given the usual clues to do with square edges and being unturned, I'd normally say this pack was from the 1860s. However, the poor cutting and the plain AS suggest that it could be younger; the illegal printing of old patterns might have continued for some time after innovations in formatting changed the legal ones.
Remarkably, given their condition, the cards do come in a box. It has a KH on the front with a completely different pattern on it from the standard KH in the pack. I suspect that either it's not the original box or it was printed abroad for sale in the UK.
It was a nice little find, all the same. If only all old packs of cards could be had for a fiver!