From memory, 25 kilo is the maximum, providing it's not an awkward object.
A bag of cement, at 25 KG, is a lot easier to lift than a table, at 15KG.
If it's a greater weight, or awkward, then it's a two man lift, or a mechanical lift, or some such term.
Anyway, that's all well and good if who your working for does it all by the book.
If your working for your average builder, and you refuse to lift, he'll probably tell you to take a high jump.
If it's causing you pain, get stronger or stop doing it.
I go to an osteopath every month, if I don't I can hardly walk.
They didn't pay me enough.
The HSE will give you guidance, but it may well fall on deaf ears to your employer.
Regardless of whether or not you have a right to refuse, if your happy to trade part of your future health and mobility now, for your excellent wages, and your hero of a boss, and a job that your happy to make the sacrifice for, then that's fine, do it.
You better make sure your getting enough in return though.