Builder's sand is usually good enough, since they have mostly stopped mining the beaches for it. Depending on the size of your dream rockery, 1/2 to 1 cubic meter of sharp, angular gravel in the same color as your rocks could come in useful to add the "scree" around the structure of the stone. Remember to include the soil that you are starting with, the slope of the site, and the sun exposure into your plans. Sudden changes in soil texture will stop drainage, so if you need to add a lot of sand, remember to cultivate the first cm or two into the bottom of the area, to provide a gradual transition between soil types. Others on this site would probably be better at suggesting plants to use, since you would probably need a far different palette than we use here in southern Arizona.

Be careful, builders sand in Britain is usually red soft sand and that is no good at all for drainage as it packs down tightly. If your soil is not well drained then what you add is SHARP sand. Personally I think it is better to add 5 to 7mm. grit rather than sand.
May I ask how big an area your rockery will cover?

Oops! Sorry, OB! Around here its probably the coarsest, sharpest sand that is easy to find in large quantities. Around here, grit is only sold in 8 oz. boxes for the budgies! : )

Personally, i'd remove at least 6" of the top soil over the entire area. Add 1" of sharp grit over the area then replace half the top soil on top of the grit.

lay the rocks in position then replace the rest of the top soil mixed with 25% sharp grit.

Get it right first time and it'll save a lot of hastles.

That is why I asked how big an area the rockery will cover. Digging out a few square feet is easy, a few square yards is a bit more of a task. It also would help to know what the nature of the soil is, clay, sand, silt, whatever. For example my soil here is a very free draining rich silty soil. I do not need to add grit to help the drainage, I add a larger gravel to reduce the fertility and to help retain moisture. I know that sounds silly but an angular gravel has lots of surfaces to which moisture clings, roots can get at that moisture.