When they finally replied to me after multiple emails they told me it is only available online, and that they would not be able to assist me further
But this was around 2 weeks ago, so things may have changed since then.

Anyway here is another website that has a lot of £1 rentals, by Hertz and other companies. Might want to check it out although it doesn't have some of the locations europcar has.
https://www.driiveme.co.uk/
I've booked a rental for July and let's see how that goes.