I come from a bilingual area in Spain and our schooling was 50/50 in both languages. I think the minimum legal requirement is a third of subjects must be taught in the minority language (which depending on each town, might be Spanish or the co-official one). I personally did sciences, geography and PE in the co-official language. Maths, history, music, art and RE were in Spanish. End of school (A-level) exams included a 2 hour written essay in the co-official language.
Welsh should be compulsory in Wales, Scottish Gaelic in Scotland, Irish in NI. Children should achieve a decent level of proficiency in them.
Everywhere else in the UK, a foreign language such as French / German / Spanish should be compulsory all throughout school. The UK is one of the very few countries that doesn’t have an MFL as a compulsory subject. Ideally you start teaching MFLs in primary schools. But for this you’d need a government that actually cares about the kids’ education.