BluePancakes That is not the typical UK roundabout... and markings can vary enormously from spiral based lanes to concentric circles to alsorts of weird flows including lanes going straight through the middle of a roundabout 
Technically there is no inside or outside lane, they are lanes 1 / 2 / etc. - however convention for a roundabout would be inside by the centre of the roundabout and outside for the outer lanes - when describing the whole roundabout, but you would describe a road user in lane 1 as on the inside of a road user in lane 2 if looking at the detail - very confusing 
as Stormtreader mentions and I gave an example above - there may be a number of reasons for someone to be in the outside lane to your left, e.g. someone coming onto the roundabout from 9 o-clock and exiting at 3 o-clock, you can not assume where someone is going...
Jaynesworld
you can do anything you like in a motor vehicle unless there is a law to say you can't
so actually Just because it doesn't say it doesn't mean you have the right to do it. is wrong - in fact because it doesn't prohibit it, in law it is allowed. The diagram is an example - as explained above, you have to read the Highway Code as guidance and it very clearly lays out when it will use wording which means something is mandatory not guidance (not in this case) and when wording is advisory (my description) and will carry weight should you do differently... this rule in the HC carries none of that wording so is the lightest form of guidance the Highway Code offers - it is an expected norm, but it is not illegal or wrong to do differently - when doing differently other aspects of the HC come into play - such as the two bits I quoted above in a subsequent rule which state that in all cases you should watch out - both drivers failed to comply with that guidance.
Verbena37 I suspect that the roundabout the OP was on will have had different lane markings - those are not normal - in which case the OP would have entered the other driver's lane... ref. someone going from 9 o-clock to 3 o-clock, depends on size of roundabout and relative speeds of their driving and OP's driving, on larger roundabouts it is very normal for the flow to mean that as you approach your exit someone is crossing, and when they entered you were nowhere to be seen...
TheWickerWoman that is great news - and definitely worth that kind of pushing on an insurance company - however I suspect they pulled out because it was more cost-effective than a share of an increasing legal bill rather than as a recognition of right or wrong. Insurance doesn't often worry about the niceties of legalistics unless it goes to court - if there is an argument that could apply on both sides, then insurance companies will often go 50:50 on the basis that it washes out across many claims - often the costs of pursuing / going to court far outweigh the costs of accepting some of the repair bill... ultimately the insurance company will look out for their costs before looking out for the OP.
umizoomi No wrong lane on a roundabout - unexpected / unusual / not normal - yes - but not wrong or illegal...
All white lines on a roundabout can legally be crossed - it is not like a solid white line dividing a road / different carriageways on a dual carriageway or motorway, you can legally drive on any part of a roundabout as long as you go clockwise
- so actually lane position is not the first or main issue here - observation / indication / avoidance of collision / etc. will all be more important issues... You can legally enter in the right lane and exit first left, or enter in the left lane and exit far right, or anything in-between - it is though your obligation as a driver not to cause issues while doing that 
toptoe not sure the other driver swerved - the other driver just continued unexpectedly further around the roundabout - still in their lane - OP drove across their lane to exit, expecting the other driver to exit as well, so sadly the OP can not claim to have not entered their lane...