“Can anyone have a guess……….”

Who knows. Perhaps they have over ordered. From this article in the Scotsman:
It’s a “traditional” vaccine: “VLA2001 (the vaccine designated ref number) consists of inactivated whole virus particles of SARS-CoV-2 with high S-protein density, in combination with two adjuvants, alum and cytosine phosphoguanine (CpgG 1018).” So it doesn’t showcase the newer vaccine technologies. Also VLA2001 is expected to conform with standard cold chain requirements (2 degrees to 8 degrees centigrade) which would be useful.

It is currently in phase 3 clinical trials with results due October. Earlier phase trial results were praised by Boris as “very promising” and I have seen reference to an 80% protection and 100% seroconversion in the high dose group. (That ref is from Valneva’s website and 100% effective always gives me pause). Valneva have stated “the UK Government alleges that the company has breached supply agreement obligations,” allegations they strenuously deny. All the other articles I have seen elsewhere say and quote the same things but no detail regarding the alleged breaches is given.

“The then UK Health Secretary, Matt Hancock, said at the time that the Valneva vaccine “showcases the best of Scottish expertise right at the heart of our UK vaccine endeavour, demonstrating the strength of our union and what the UK can achieve when it works together.”

So much for that.
It’s a French company.

Given all the other stuff going on such as the N.Ireland withdrawal agreement argument and the channel crossing migrants etc etc maybe they wanted to give the French and EU the middle finger.

Until we know more about the breaches of contract it’s guess work. Valneva’s website blurb on this vaccine.