sirfred is right though.
The NC says that every child should be able to swim 25 metres by the end of year 6, and the school is supposed to ensure that.
I am an ex teacher and have 3 kids through different schools, and yes, most schools have swimming lessons.
BUT how they deliver them is very different.
eg
school 1, all kids in year 3 go to swimming pool on week one. Those who can do 25 metres do not return next week, they don't need lessons. Those who can't have lessons until they can, so they drop out as soon as they can. (hated this system)
school 2, small school, all of key stage 2 goes in summer term every year.
school 3 every year group goes from year 1 -6, but only for 6 weeks each year, so the school has a swimming slot and rotas its kids.
school 4, swimming done in year 3 and 4, for the whole year , once per week. If you join school in year 5 and can't swim, tough.
pros and cons to all of them, but school 4 is the best in terms of learning.
As to the OP. This is possibly the least important thing on the curriculum to chose a school over (and I am a passionate supporter of school swimming)
Mine did after school lessons during the week (saturday morning is sacred in our house)