The Isle of Skye in the west of Scotland is truly spectacular.
The Western Isles (Outer Hebrides)are less hilly, but very interesting and have challenging weather. You can go from Ardrossan on the Scottish mainland to Arran, then to the Mull of Kintyre, then Islay, or ride up the mainland to Oban, then ferry to Mull and Iona and on to Castlebay on Barra, then up through the chain of the Hebrides to Stornoway, ferry back to Ullapool, and then ride via Cape Wrath to John o'Groats. You can take a little ferry from John o'Groats to the southern tip of the Orkneys, and ride through them (easy, very interesting Neolithic, WWII sites and a cathedral), then take another ferry to Lerwick in the Shetlands and ride up to the northernmost point of the northernmost island in Britain, Muckle Flygga on Unst. I did all that on a Brompton (folding bike) in 2009, taking about 2 weeks from Ardrossan to Unst, and had a whale of a time. You need a good tent that won't blow away in a gale, though, and good rain gear.
If you really want to climb, some of the best rock climbing in Britain is in Wales, especially in Snowdonia, North Wales. There are some good steep roads there for bikes, too. But the steepest road hills in Britain are in Cornwall and Devon, at the south end.