I've been told by this agency, which I thought was the top UK Commercial Agency, that I will need to have test shots done by them to be considered for their books.

I'd always been told never to pay money upfront to an agency, but I've also been told this agency is the best for Commercial Models in the UK...so what am I to believe?

That  agency has been about for over twenty years was started in Norwich
I have used them as a photographer looking for models they class you paying training as paying, to be registered with them is like any other I heard from the models I worked with good and bad,  the models I hired where very good standard,
on the negative I still do wonder why they feel they have the right to charge the model and take a percentage from any booking as I found out it was up around 40 % of the total cost, 
so they are in my book a good agency I if wanted to pay a lot of money for a model would consider hiring from them without contacting any other agency as I went that road to start collecting images for placing into stock libraries,  know I feel I have improved my photography alot and so dont need to go to that extreme,

so yes they are legit but as with any agency they after after sharing your income and certainly dont like it if you have other avenues or that was how I understood from talking woth the models