you’ll not get through 5mm plate with a nibbler I would have thought (not my hitachi one anyway, that won’t do much more than 1.6mm) you also won’t get a nibbler from screwfix (and for much less than £300 – £400 anywhere else)
If you go down the hole saw route get the bosch bi-metal ones. they need their own style of arbor, but they’ll outlast the cheaper ones by a massive margin, in fact I don’t think I’ve ever blunted one. If there are a lot of holes to do they’ll still be a ball ache and the noise will drive you mental, and working with a drill driver will result in lots of wrist-spraney moments. A 9 inch grinder and good disks would give a faster, clumsier looking result. Neater work could be done with a 4.5″ grinder and a good supply of 1mm disks and a gentle, patient hand.
An evolution TCT saw would have made a better job of cutting the end of the tank of, you can’t really plunge cut with them really. If you used a grinder to start things off you could use an evolution saw to cut sets of long slots rather than small holes. Shop around as there was some pretty heavy discounting going on recently. B&Q might have them