At only 3.5K per year it probably isn't worth converting for you, or at least not unless you'll be keeping it long term.
Dunno about 'done by someone close by but reputable', I convert more Elgrands from all ends of the UK and more than any other firm. 'Going out of the way to buy LPG' has been mentioned, the best way to not have to go out of the way is to have a big LPG tank fitted, I fit by far the biggest. I personally go less out of my way to buy LPG than an average LPG owner goes out of their way to buy super unleaded, but I have ability to easily fit a quick release extra LPG tank which increases range on LPG to 700 miles, twice as much range on LPG than on (super unleaded) petrol.
If all LPG forecourts closed I'd still be running my cars/Elgrands on LPG, I'd be having it delivered to my door in 47kg bottles for £70 per bottle. If all forecourts that sell petrol close it's unlikely anyone will be having petrol or diesel delivered to their door in bottles or be allowed to store petrol in bulk (500 litre plus usually 1000+ tanks), people are allowed to have bulk LPG tanks at home aka LPG home heating and that will be the case even if/when most people drive an EV and most petrol forecourts have closed.
I think whether an LPG conversion increases the value of a vehicle depends on a number of factors including how good an LPG conversion the potential buyer has been fitted. I'm the most recommended LPG convertor especially for Elgrand LPG conversions. Same question again, if you were looking at 2 identical Elgrands but one would cost half as much to run as the other in terms of fuel costs, which would you buy and would you be prepared to pay a premium for it? If you wouldn't pay a premium it's unlikely you'd be able to buy it because it would be sold to someone else who would pay the higher price.