I wanted to thank everyone for their advice - and decided to post because I keep getting PM's and emails due to this thread!
I did not strictly cave while there, I went with two friends from my St Joseph Valley Grotto days who are also avid travelers. We rented from Icelandic 4x4 (Definitely rent a 4x4, definitely that company) and had a 10-day trip (really 8 full days there, as cap days traveling). We did Golden Circle and Ring Road, with a few explorations off the normal. By few I mean, you'll see roads and wonder where does it go and we were the type who took it as far as we could safely, to see! Hence, I disagree with those who said a 4x4 was overkill for doing just ring road. Despite staying relatively close to it, the few attempted mountain passes we did it was VERY helpful and it allowed us to get closer to the glacier tongues on our own. Definitely get the gravel/sand/ash insurance, because your vehicle will get sandpapered in the wind, and you will get multiple windshield cracks. PAPER MAPS. Even the rentals GPS was shit.
Classic 4x4 road...
Road Less Traveled by
Amy Hinkle, on Flickr
Mountain pass (this is MAIN road, at 12% grade in spots)
Mountain Roads in Iceland by
Amy Hinkle, on Flickr
And here is where we really got stopped, by snow cats, and decided oh what the hell it is just a 3 KM uphill both ways on steep canted ankle to knee deep snow to where we want to go, and we hiked it!
Snow Cats by
Amy Hinkle, on Flickr
People ask me about cost...well we split the rental and petrol 3-ways so that was about $300 for each ($600 per person for vehicle in total). Hostels were $30-$45 a night and were cheap good accommodation. We ate out maybe one meal a day, cooked the rest - brought enough camp food to use just hot water in the hostels, a smart move as most towns despite saying open until 6pm or 8pm, closed about 4pm! So don't count on groceries, although if you can it is the smarter move than eating out.
I'll let you do the conversion from dollars to pounds.
With WOW air now operating, I did notice flights round trip to London from Iceland were as little as $200! I was very very tempted to pop over to say hi to ya'all for a week, but it did not pan out.
A lava cave (Vatnshellir)...
Walls of Lava by
Amy Hinkle, on Flickr
An ice cave we found exploring on our own near the base of Fl?aj?kull
Sharon in Ice Cave in Fl?aj?kull by
Amy Hinkle, on Flickr
All my Iceland photos:
https://www.flickr.com/photos/sunguramy/sets/72157666940703732