Due to the bad weather in Spain, the supply of tomatoes, peppers and cauliflower, among other things, is now much less than usual. In British supermarkets, consumers therefore have to deal with empty shelves in the vegetable department.
Buy up to three tomatoes or peppers at a time. This is now the case in British supermarkets. Much less vegetable comes in from Southern Europe, so that there are shortages on the our side of the sea.
In the UK it is a bit more difficult to get supplies from elsewhere, because it is an island. In addition British supermarket chains do not want to raise prices sufficiently either. If that were to happen, consumers would be more likely to switch to other, cheaper vegetables.
Thirdly, if Spanish companies have the choice and pay customers the same, they will prefer to export to other EU countries rather than the UK, because it is simply less of a hassle than exporting to a country that has just left the EU.