Simplistic nonsense Pete.
As of our total current exports, 44% of it total goes to the EU. Tariffs would reduce our exports due to some of the goods becoming uncompetitive in that market. Therefore our output will fall, which lowers GDP and reduces the treasurys purse pro rata.
The argument that we import more than we export as a total therefore we gain and the EU loses, also ignores the fact that the EU is a collective. Germany for example only relies on the UK for circa 5% of it's exports, and so any net reduction in sales to these shores has a lesser impact on them, than us losing a chunk of our 44%.
Not having access to the single market would also decimate our financial sector, despite the mutterings of bumbling Boris that passporting will remain, this is far from a certainty if we don't agree full access to the single market.
The bluster over a tariff deal being an acceptable alternative is just that - bluster.
And it overlooks the huge importance of services to our economy, which aren't covered by the WTO at all. We currently have the financial passport scheme, and it's hard to argue that we wouldn't lose significantly were we to lose that.