Does anyone have any experience of travelling from Zurich to the London by plane with a small dog post brexit? Is cargo the only option or can you travel with the dog in the cabin?

Will depend on the airline and the size of the dog usually. See this as well though:

https://www.blv.admin.ch/blv/en/home...frettchen.html

There is no change in the rules due Brexit. Think dog plus carrier must normally be below 8kg.

Again nothing to do with Brexit, but the UK won't allow animals to enter the country by air except as cargo