As a professional musician who also did weddings in my student days and have many colleagues who do this for a living now... I must say, any quartet charging you £200 for a wedding (in 2021) is either one of extremely bad quality, or they are massively undercharging everyone else in the area, and therefore making it impossible for others musicians to earn a decent living. Please don't hire such 'musicians'.
Ceremony + wedding breakfast - I would say £800-1000 is the very minimum they should charge.
You are paying for:
Being there and actively working for 2/3 hours
Set up and pack up - they will most likely need to get there 1h before guests start arriving, so that's another 1-2 hours of their time.
Transport to the venue/s
Preparing what they're going to play. People often brush this off but they will need to meet up once and rehearse - or at least read through - what you've asked them to play. If it's a new item not on their usual rep list, someone will have to sit down for a few hours and find the music, arrange it for the ensemble, send it to the others, who will then need to learn it.
Plus all the hours one of them will spend emailing or calling you, and going back and forth between rep options, dress code, timings, etc. Yes this takes hours, never have I met a couple who managed to decide all the above in 30 minutes and one swift phone call 
You're paying for 4 people to be available for half a day, and a good chunk of the week leading up to the wedding.
Most ensembles and musicians will have a base fee for the shortest amount of time (ceremony only), which will seem high to begin with, but it doesn't exponentially grow if you book them for more hours.
As a soloist I used to charge £300 for ceremony only, but only £400-450 for ceremony + 2h at reception. So £150 for the first couple of hours, and only £50-75 per hour after that.
It basically depends on how long and what you want them to play, the location of the venue and distance from where they're based.