I've probably clocked up getting on for 1000 nights sleeping in the rooftent on my 110, mostly in Africa and the Middle East but also several hundred in the UK and I've experienced night time temps ranging from 30 degrees plus to 10 degrees minus and serious downpours.
For quick set up and packing away camping in the bush or desert with the comfort of being up away from the creepy crawlies and big beasties (though this is mostly psychological as insects, snakes and most predators are perfectly capable of climbing onto a vehicle if they really want to), you can't fault it but for use in wetter, colder climes, the benefits are less clearcut.
With your tent on the top of your vehicle, you will have to strike camp if you want to pop to the pub, shops or go out for the day and put everything back up again on your return and if you are camping on a "normal" campsite, you may risk losing your pitch if you don't leave something marking your spot.
In addition to the roof tent, you will need some kind of awning to shelter you from the searing UK sun or more probably drizzle and rain. You can improvise something with a tarp but if you are looking for low hassle camping then you will need to budget for one that fits onto the roof rack. I have a simple canvas awning which folds out from my roof rack but there are some quite fancy "bat wing" awnings available now but all add to the cost.
Many cantilever style roof tents have "skirts" which use the overhang to create a tent which can be used as a changing room, bathroom, cooking area or just somewhere to sit but this is again something else to put up and take down.
Unless you are planning to pipe in warm air from a Webasto/Eberspacher type diesel heater or use a 240v fan heater, roof tents can get pretty chilly at night and you obviously can't cook in them or have a campfire. The two nights I spent in mine in Buttermere in December in well below minus 10C were two of the most miserable in my life!
Think about storage - mine lives on top of my 110 year round which is not ideal and I have to air it regularly but several people I know have opened their out after a winter stored in a garage to find that mice have trashed the canvas.
A heavy roof tent (the MyWay that I have is comparatively lightweight but some weigh a ton) can negatively affect the stability of a vehicle and exceed the recommended loading for the roof gutters and/or roof rack. My 110 has HD but standard height suspension and a HD Brownchurch galvanised roof rack with 14 points of attachment to the vehicle and my roof tent doesn't make a great deal of difference to the stability. A heavy roof tent on raised suspension may not be as stable and some light weight roof racks with 6 points of attachment may put too much pressure on the roof gutters - I hired a 110 in South Africa with a roof tent on a flimsy roof rack and you could not open the doors if someone was in the tent!
Roof tents used to be a real rarity in the UK and most of the original ones were either from South Africa (Hannibal, EeziAwn and MyWay) or Italy (Maggioloni/Autohome) but there have been quite a few new entrants to the market and a lot of Chinese made clones.
There are two basic designs;
Cantilever tents which fold out to hang off the back or side of the roof rack and which typically have a soft waterproof cover.
and
Hardshell designs which fit entirely on the roof rack and open either straight up or are hinged at one end. These are probably easier and quicker to put up - they either just rise up on gas struts or get cranked up with a handle.
I'm not familiar with the iKamper but that seems to be a combination of both designs. Check that after sales support is available for any replacement parts you may need.
I bought my roof tent direct from the factory in SA nearly a decade ago and am not up to speed on current prices in the UK but £3k seems a lot.
http://www.mywayrooftents.co.uk/myway-roof-tents-product-information/SERENGETI-10/
You've just missed the Overland Travel show at Stratford Racecourse in September and most of the Land Rover shows are now over for the year but these will have lots of dealers displaying their wares. Even if you don't own a Landy or have plans to head off over the horizon with your roof tent, I'd strongly urge you to go to a show and look at as many as you can and ask to try putting them up and down and think carefully how it would work with your vehicle and whether you can reach everything easily. I originally had mine opening over the side and hated putting it up as I had to get onto the roof to undo the cover, hanging it off the back transformed the process and I can have it set up in around 3 minutes.
Probably more information than you want or need - I'm quite passionate about overland travel and have definitely had VFM from my roof tent but I've seen too many people (usually Landy owners) get seduced with the idea and end up wasting their hard earned on something that just doesn't suit the camping that they want to do or in more than one case having a significant other who flatly refused to climb up the ladder!
Good luck with whatever you choose.
