The major thing to point out is that the male guppy is likely to harrass the female to death if kept as a single pair.
In those terms, it would be best to have at least 2, and even better 3, females so that the male splits his attentions across the females, and doesn't focus just on the one.
However, 4 guppies, plus the neons and pygmy cories in a 10 gallon is very overstocked. If you wanted to do it, I would strongly suggest getting a second tank just for the guppies.
The other thing to consider is that it won't be easy to move the guppy fry on. Local fish shops already have a trusted source of fish, which are probably costing them 50p at most. Why would they risk allowing in diseased home-bred fish - I'm not saying that your fish are diseased, I'm saying that the LFS don't know whether yours are or not. You could try Gumtree, but then people won't be interested until they are at least 3 o4 4 months old - by which time you could have 90 or so fry, at various stages of development.