I don't think it's Covid or Brexit related, it's just vets being able to charge whatever they like.
I bought a lifetime vaccination plan when I bought my dog, it was about £100. This gives my dog his annual boosters every year for life at no extra cost to me. My dog is now almost 8 so I've definitely had my money's worth from this.
I do take him for kennel cough vaccinations each year. In 2018 it was £25.50, 2019 - £27.00, 2020 - £37.00! He's due again this month and they've quoted me £43.00, so I've shopped around and found another local vet charging £33.70, it's still ridiculously expensive but has to be done.
Some of these monthly plans are really pointless. Taking one plan previously mentioned at £16.99/month (£203.88 annually) for annual boosters, 2 general health checks and 10% off any medication. Clearly I don't pay for annual boosters but I imagine they're no more than £60, my vet does general health checks every 6 months for free, so to me, this means you're paying at least £140 per year to save 10% on any treatment, that means you're expecting to be receiving at least £1,400 worth of treatment every year (not including the consultation fees). You're only benefiting if treatment costs more than this. Even then, this is in addition to any insurance you're paying, which basically means that it's your insurance company that's benefitting, not you because the cost of your treatment (minus your discount) is reported to the insurer and they pay that, you don't get the 10% back.
Plans that include flea and worming treatments are also seriously overcharging, you can get these treatments online from any one of at least 8 common vet medication stores for a fraction of the price. I buy 3 months of flea and worn treatments for a medium-sized dog (15-25kg) for around £20 and it gets cheaper if I buy 6 or 12 months at one time.
People need to do their homework before signing up to these plans with their vets.