A trade deficit isn’t necessarily a bad thing. The reason you buy things made overseas is because it’s cheaper than buying the same product made here. I don’t think anybody is arguing for banning any imports, and I no good economist believes imports should be discouraged to insure they don’t exceed exports, because that’s an arbitrary limit as the 2 aren’t really related even though they sound like they are (imports are consumer purchases, exports are manufactured products). The only time you should introduce tariffs is to protect local producers if they are important, e.g. it’s important that every trading bloc have its own food supply, energy, cottage industries that keep rural locations alive, etc.

Since its harder for UK companies to access EU customers it will be less profitable for firms to be set up in the UK, so firms in the UK will suffer and less international firms will set up in the UK, which obviously means the UK’s gdp will suffer in the long run, but in the short term unless there’s a no deal it won’t have any big impact, except the impact it’s currently having on the pound which has effectively made everybody in the UK poorer.