I'm started to look at fitting winter tyres when they come into stock (fronts are starting to wear anyway), and have noticed a lack of choice with my tyre size (235/55 R19). However, there seems more of 255/50 R19 available, and maybe a bit cheaper. There's only 0.5% difference in overall diameter (according to a converter I found online), but has anyone made this switch before? Any potential problems with a wider tyre?

Only problem I can think of is a wider tyre could rub against the suspension and steering. Which is also why you cannot fit snow chains on a Freelander with tyres above 17 inches. Need a tyre expert to advise fully.

Is there any legal problems you could have with your insurance etc, fitting wider none standard tires, 235 to 255 ??
because I have thought of doing the same.
I know they will fit and work, but could your insurace throw out your claim (you know what the are like for not paying out)

With insurance it's probably best to check to be sure, as you say, in the event of a claim they will simply use the info as a means of saving a payout.
If you have a concern that it might be a problem, Sod and his law will dictate it will be a problem.

Re the wheels, chains can only be fitted to 16" wheels, above this size Land Rover use the spyder device.
At 17" there is only 10mm clearance to the steering arm if I remember rightly and standard chains won't work, given that 255 tyred will be 10mm wider each side they may well rub as rchrdleigh states.

You be better off with another set of 16 or 17 inch wheels rather than run the risk of damaging 19 inch alloys. You can then go up to 65 with 17's.
I run Nokian WR2's all year round on 17's. No kerb damage  Another member of the failed FL2 clutch/DMF club, twice.