Paper road atlases designed for exactly this are usually pretty widely and cheaply available in the UK from stationers such as WH Smith or Waterstone's, or any of a number of independent local retailers. You could probably buy one in advance from your favourite online retailer to be delivered to your UK destination, ready for you. A nationwide atlas is likely to be perfectly workable and not that expensive (under GBP 10); I've usually only bothered with county-specific atlases (note, as another poster said, that the sites you're looking for are in Bristol, not Dorset) where I need fine detail of residential areas.
All that said, I wouldn't buy one and haven't in years. Waze is an excellent free GPS navigation application with crowdsourced traffic data and more up-to-date maps which will make your journey much smoother. If you're concerned about data charges, Google Maps is also excellent for satnav and can be used completely offline if you download maps in advance - it doesn't take much device space for a really quite large area, and I traveled round northern Scotland that way last year with almost no signal but excellent navigation.