To be honest, I disagreed.

First of all, I would've assumed that the original design files should not be sold or even be given out, as long as being mentioned first hand well within the contract. Which comes down only given to the client of what's the "final" product or outcome to the client "only".

If you have designed the model yourself from the ground up, technically it may will be yours, be legal rights, as for the client, he or she should only have the rights to use it "as a design-based outcome", rather having the total rights to re-use or re-sell it - because you're the original creator. Otherwise, the client should well spend few times of £120 (£120 x 10 or more times, for example) for your original "editable" model, if the client insist and wanted to get hold of your original 3D editable model.

Peace.