The clean air zones will get all Defenders sooner or later, you'll spend £6,000 and then they'll tighten the laws, my advice would be to sell it and buy something that will be exempt.
One thing, Defenders still sell well so you'll not lose any money on it.
However if you love your Land Rover and decide to get the work done, make sure before you spend the £6,000 that it will then pass the ULEZ.
lf you can't be sure, or they won't tell you, l wouldn't take the risk. These people are like the DVLA, they won't budge for anything.
My brother lived in London and had the same issues with his VW Camper conversion. He spent a fortune on a whole new engine and they still wouldn't accept it.
Don't forget, they don't want these vehicles in the City at all, so nobody will be on your side.