Is it the Homebase stuff? If so, I cut mine with a hacksaw but it did take a fair bit of perseverance with the saw. Two options I can think of:

Put a new sawblade on your hacksaw (although thats only really necesary if your current one is a bit blunt)

Or, invest in an angle grinder - there are some for £20 in the likes of Argos etc that would be fine for cutting through the box section.

I suppose there are other otions like flame torches and laser cutters but that would just be me being complicated