If I have to give an educated guess I would say it's due to the logistics of maintenance for the Eurofighters.

> The Eurofighter’s European collaboration model distributes manufacturing and support across partner nations, and this model lowers acquisition costs for the initial adopters. However, the Typhoon's maintenance requirements, especially for its EJ200 engines, can be complex for operators outside Europe, which has hindered export sales.

Given the UK has left the EU, I wonder if that would effect their maintenance costs and availablity, although I would assume something could be worked out given they're in NATO!