Brexit crushing small business while leaving the larger businesses relatively unscathed was part of the plan all along, not just in regards to beer. The big players always knew they could deal with added fees and paperwork by passing on the cost to the consumer because they were already part of big pub chain and franchise monopolies.
Many pubs go though a hight turnover of landlords as they are all run as franchises now, with landlords forced to buy from the owning brewery while also paying rent to them, but with margins so thin and brewerys setting the prices they can't survive unless the place is always busy. So they quit and then in then another hopeful steps in to repeat the process. The brewery don't care, they still get thier money.
The Independent ones that were quick to react stopped selling to pubs and started selling to supermarkets where margins were better. Either that or they caved in and sold the brand.
Now the multinationals have successfully crushed craft beer, the threat to thier profits that was showing people you don't have to drink washed out larger that they keep putting a new name on every year to make it sound better.
The new tax rules coming out that penalize higher alcohol content is a further step towards the dominance of cheap, mass produced, piss water. More work being done by the conservatives on behalf of thier corporate paymasters.