Exactly, I chose to take British nationality- because I arrived in UK before the EU - and had to to remain and work, etc. We 'had to get married' too - not because I was pregnant ... but that was the only way we could stay together.
Had I arrived after the EU, I probably would not have done so, as there would have been no need or incentive to do so - as rights, etc, were protected.
BTW in Switzerland, UK citizens who live here either have a short permit, a B permit or after 5 years, a C permit. C permit holders can vote in local elections, but not in Federal/national elections. The Maire of my Commune (Council) is NOT a Swiss citizen. He was born and bred in Belgium, has lived here for over 20 years- and was elected a Maire, but has chosen not to take Swiss nationality. Doing a great job too.