Depends how much you wanna spend/what tools are already in your possession
Either oxyfuel or plasma will be the least effort, just use a guide to cut against unless you have inhumanly steady hands
Oxyfuel is fairly cheap to get set up with, it just has the enduring cost of rental and refills on the oxy. The best thing is you already know someone local with a set, everyone does, ask about in the pub/local hardware supplies/farm supplies and see who can lend you theirs for an hour or two
A plasma cutter is a significantly larger investment, especially since you also require a compressor. Even looking at low end Chinese plasma's and diy compressors you will be north of 300quid, not exactly have a versatile setup, and suffer from low run times dictated by the compressors output. The plasma cutter is arguably the most useful of the three options here though
A 9" grinder will go through it no problem, and will give a fairly neat cut, you would probably use a couple of decent quality discs. Every workshop should have a 9" so if you don't have one yet then it's about time
You could probably get a diy spec grinder for circa 50quid or a decent one for around £100
Personally I wouldn't consider the cut on a 4.5" grinder, but it is doable, just time consuming, noisy, tiring, and heavy work for the tool and consumables