Lorry's "fault" for indicating right approaching roundabout is debatable as he/she changed mind and indicated correctly left when on the roundabout. We all get lost at times and end up in the wrong lane, or realising we actually need the 2nd exit instead of 3rd... So, it's a little poor to miss the left indicating, but I know in that situation it could be easily done.
But, the lorry also exited from the inside lane on the roundabout without checking mirror - this is even more frustrating/wrong as they knew they were incorrectly indicating right prior to the roundabout, so they should have double-checked for anyone misinterpreting their erroneous signalling. He's technically changed lanes from inside to outside and then left the roundabout, all without looking. See it happening quite often when the exit from a roundabout is dual carriageway and someone on the inside lane just roars to exit, while someone on the outside lane is still wanting the next exit. Whoever is on the inside needs to check mirrors and give way to traffic on the outside, as they're doing a lane-change. I fully appreciate, with the size/visibility of a lorry, it's expected for them to take this line around such a roundabout, when going straight on - this still doesn't mean they shouldn't check when exiting tho...
I think the lorry technically did more "wrong" but experience would make a driver not to be on the outside of a lorry trailer around a roundabout and thus get into that situation. I could almost see myself doing it, especially if it's busy traffic and other cars were entering the roundabout from both lanes - if I saw him indicating right prior to the roundabout, I probably would have been beside him as he entered the roundabout. Depending on the tightness of the roundabout and whether i thought he needed to use both lanes to turn around the roundabout, I would have either gone with him or waited. But, I guess that if I had entered the roundabout at the same time, it would/should have made me a lot more visible to him, before he exited...
Imagine if the lorry had entered the roundabout from the right, intending to go right (ie, entered from the east, intending to exit on the north) and the OP joined from the south. Then they both would have done the exact same manoeuvres, in the exact same lanes, and with the exact same indicators, would you then say the lorry driver did nothing wrong?
You say the lorry driver should have checked, but did you expect him to stop on the roundabout, blocking it when the OP was behind the back of his trailer? Or would you have expected the OP to not go for a non-existent gap on the exit of the roundabout?
In the end, as the driver behind, you have more visibility of what's happening in front of you, you are further back on the road, and have to make allowances for vehicles crossing in front of you for an exit, even from the inside. Especially so for large vehicles that have to take wider lines and fill up more of the available space. If there's only one lane marked, there's little to make a driver think throwing it up the inside is viable. You can see it's a very small exit leading straight into a single lane, ie it's only designed for one vehicle.
Holding off on attempting to blast around the outside of the roundabout for a couple of seconds would have allowed OP to realise what was going on, and allow him to gracefully slot in behind the lorry on the exit.
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