When we travelled to Britain with my 17-year old son two suimmers ago, we each had "destinations". He chose such places as Loch Ness, Whitby (for the Dracula connection), the Hard Rock Café in London, a play with "somebody famous" in it (we saw Cate Blanchett in "Plenty") and so forth. Your daughters would no doubt make different kinds of choices, but I think it'd be important for them to have some things that *are* their own choices; it would also make them more open to finding enjoyment in the adults' choices, plus it would encourage them to do some research on the country before they go.
We had a rental car and so were able to be spontaneous -- many of our best experiences involved picking things off the road map, or from roadside signs (Cairnholy in Scotland and High Rochester in England, for example). Also, our son became the official navigator, sat in the front with the road maps and guidebooks, read to us everything about every town we came to, and so forth. It was a very formative experience for him (and he did a much better job than his mother could have...). Despite the younger ages of your daughters, I would guess that you can find similar ways of making them feel involved.