A few other things to consider - mummy (tapered) or square? Do you want a cotton feel to the inside? Do you want to be able to zip the two together?
Generally speaking a 2-3 season has a comfort temperature around 3*C. That's the best comparison really - manufacturers use some imagination when it comes to season ratings and the "extreme" temperature is useless.
There should be plenty of choice around the £25-30 mark. Vango, Outwell, Coleman, Snugpak are often recommended. They'll probably be made in China so whether the brand is important is another question.
For example (just picked at random, not a recommendation):
I can't seem to find a comfort temperature for the Wilderness 250 bag, but "suggested usage" seems to be 8-20*C. Which may mean it's untested. In my opinion that's not warm enough for mid-September onwards when days can be warm but a clear sky can produce a light grass frost before sunrise.
The Nitestar 250 has a comfort temperature of 4*C, which is just OK, as does the Wilderness 350. The Nitestar 350 is 2*C, which is a good 2-3 season temperature.
If either of you tend to feel the cold, or the further north you camp, it wouldn't harm to go for the 450s if you can find them in your budget.
As to leg room in mummy bags. No, there is no leg room. Typically you'll be able to get your feet 12" apart but you can't lift one knee up and leave one leg straight. Both legs have to do the same thing.
A square bag rated at 4*C is just as warm as a mummy bag rated at 4*C. The mummy shape is used to reduce weight and pack size.