I would say your unlikely to find one in new condition, and I wonder why you would want one? To change to a larger one the circuit needs to be suitable, so heavy enough cable, not only to take current but also ensure the loop impedance is low enough. Type B MCB is x5 not got an old enough copy of BS 7671 to look up type 1, so 230/(32*5) = 1.4375 Ω x 95% =
1.365625 Ω with a 45 amp = 0.971111 Ω cheap plug in testers even with loop only go to 1.9 Ω so you need an expensive loop impedance meter to test.
Also question of how you will get RCD protection?
So to my mind alarm bells are ringing, why would you want to change to 45 amp?
I don't think the distribution boxes that took those fuses were consumer units, so not permitted if under the control of an ordinary person, so looking at a commercial set up not domestic, so then HSE comes in, and also minor works certificate etc. I would not want my name on the certificate I would simply tell boss needs a new consumer unit before it can be changed.