Cheltenham is lovely. We moved in November.
Primary schools vary in size. Some are single intake (so around 30 pupils per year - 200 children to a primary school). Others have two, or even three classes of around this size per year. Many schools have around 60 children per year.
Our school system starts with reception (children who will turn 5 that academic year) and then the remainder of primary school is years 1-6. Some schools are just infant (reception and years 1 & 2) or junior (years 3-6). Often an infant and junior school will, though separate schools with separate management, share a site. An example would be Benhall School and St Marks Junior in Benhall.
Senior schools is years 7-13. Years 10-11 are GCSE and years 12-13 are A Level (or equivalent). Both are two year courses, so it important to try and stay put for the duration. Some schools run for the whole of years 7-13. Other schools stop at year 11.
The naming system for UK schools changed about 20 years ago and many people still refer to some of the years by their old name. Most commonly, years 12-13 are called 'sixth form'.
If you attend a school that stops at year 11, you will need to attend a 'sixth form college' for years 12-13 (assuming you decide to continue in academic study). Some children also decide to go to a college even if their school has a sixth form, for a variety of reasons. There is also some movement at this level more generally. For example, my female cousin attended a sixth form at the local boys school (which had a mixed sixth form) to get the subject combination she wanted.
Most state schools will only do a very nominal amount of foreign language at primary, if at all. I wouldn't worry about that. Schools vary a lot on music lessons - some have more than others.
Does that help a bit?