Our own miniscule domestic production will be a leading factor.
Of the three countries mentioned:
Spain produces 4,768,595 tonnes of tomatoes a year.
The Netherlands produces 910,000 tonnes.
France produces 712,019 tonnes.
The UK produces 66,893 tonnes.
That data may be slightly out of date, but we've got a larger population than all 3 countries, yet our domestic growth is non-existent in comparison.
France and Spain you can at least explain with their Mediterranean climate, but the Netherlands are clearly making a concerted effort to produce their own within greenhouses.