It depends on how much the gap is. On grand houses, the type with 10in and deeper built-up skirting (often multi part), it used to be normal to fix timber grounds to the masonry before plastering, and for the plasterer to work to the grounds. On lesser houses this was never done, simply on grounds of cost, and it still isn't. When I install skirting onto a traditional plastered wall (i.e thick plaster onto masonry) I generally have an electrician's bolster to knock off the high spots, but even so there are places where decorator's actylic caulk will be necessary. Modern dot and dab boarded or dry lined walls should be nowhere near as bad. Providing you don't need more than 4 or 5mm of caulk over part (not all) of the length of the board then I'd regard that as reasonanly acceptable
PS I also live in a terrace house (or an "artisan's dwelling" as estate agents darn sarf call them)