After digging up some bamboo plants from the garden I want to now put it
into pots. I thought it best to use garden soil mixed with some compost to
make the pots lighter to move about.

When I went to B&Q they have compost at 5 pence a litre and growbags at 3.7
pence a litre. What is the difference between the two, and would the
growbag contents be just as good for this purpose? Thanks for advice.

It is simplest to have a soil-based growing medium for bamboo, ie a mix of soil and compost or a soil-based compost. Like all grasses, but few other plants, bamboos need silica, and they can't get much of that from a purely plant-based compost. People do grow bamboos in soil-free media, (which I think maybe gives them better control of the conditions for propagation purposes), but then they have to add special stuff to give them the silica. I suppose you could just throw some sand in, but I think the silica would be easier to uptake from the silts and clays in ordinary soil than from sand. (Yes I know not all sands are silica sands, but the ordinary stuff in the shop is mostly silica. And some soils are silica deficient too, but they are likewise uncommon.)

I think growbags are generally designed for growing stuff like tomatos and cucumbers, and so would have more phosphate and less nitrogen than general composts. I think bamboos like more nitrogen, and high phosphate would be wasted on them.