There is no such thing as right of way in British motoring law - it is priority, that is a significant difference in the event of an accident.
White lines are generally guidance to assist, but not always mandatory
A person in the left lane of a roundabout is allowed to keep circulating the roundabout in that lane for as long as they wish - nothing illegal in doing that
The driver within the inside lane is crossing another lane to exit e.g. with a 4 junction roundabout at 12 / 3 / 6/ 9 if you are going from 6 to 12 as described someone could also be driving from 9 to 3 in the inside lane why would you feel that you have priority over them?
2 lane exits from a roundabout are so that when two cars exit simultaneously it can take the traffic load. In the UK a right hand lane is only for overtaking, unless marked separately (e.g. as left / right / straight on only). Therefore in entering in the right lane, travelling through the roundabout in the right lane and exiting in the right lane, the OP should only be doing that if using that lane to overtake cars doing the same in the left lane - otherwise, if there is no need to overtake the OP should move back into the left lane prior to exiting the roundabout.
When overtaking the person doing the overtaking should give way to the person being overtaken unless there is a change of lanes, in which case the person changing lane gives way to the person remaining in their lane. At all other times the person changing lane gives way to the person already established in that lane. The OP was crossing lanes and didn't give way to the person already in that lane.
The other driver may have made a valid assumption that in choosing to enter via the right hand lane the OP would be turning right - why was the OP in that lane...? You can use any lane, but if you choose one where it might confuse someone else there is even more obligation to ensure that you prevent confusion / avoid accidents...
There is an expectation that all road users should do all they can to avoid an accident
So, sorry to hear about the accident but while a common misunderstanding, actually the OP's use of the roundabout has some misconceptions. Yes, she wasn't necessarily to expect the other vehicle to continue, but she did make the assumption that it was going straight on - so a scenario where the other driver's action was unexpected and the OP didn't allow for the unexpected / didn't observe the niceties of changing lanes / crossing their lane / made an assumption etc. - 50:50 sounds quite likely.
Ultimately many people use roundabouts very badly, there is also an inaccurate assumption in the UK about rights in motoring which perhaps leads to further assumptions about how others will behave - instead, a part of driving should be to not make assumptions and to allow for other drivers to possibly make all sorts of moves!