There's a document produced by DEFRA which should answer the legal needs: Guidance note for the Control of Pollution (Oil Storage) (England) Regulations

If I was in this situation I would be recommending the IBCs were stored on bunded pallets. Not for any legal reason, but purely from the point of view of practicality and to a certain extent safety. Presumably you decant from these IBCs and in this process I am guessing there will be a small amount of spillage due to drips etc. With a bunded pallet that will go into the bund. On a normal pallet it will go on to the floor resulting in a patch of slippery oil that people can walk over, spread around the factory etc. On a bunded pallet the whole area is safer, tidier and easier to manage

One challenge if you bund them is how to move them in and out - which I presume you do, perhaps by FLT? So another option is to design the storage/supply locations to be self-draining to a 'hazardous drain' that includes an interceptor pit. You'll then have to have arrangements for pumping that out and disposing of the spilled liquid - but you'd also need the same for a bunded area. Also, for any bund you need systems to ensure the drain valve is secure, but also to use it to drain off rainwater that will accumulate. Redesigning the sites may be too expensive for an existing installation, but is probably the best option for a new one. There can be a low bund on most of the perimeter, but a suitable low-incline ramp on the in/out route. Bunded pallets sounds like a good, cheap option too.