Within the UK, Royal Mail Special Delivery is the best IMO, largely because it's easy to collect from a local depot or Post Office if you're not in, don't have/want to use a work address and keep irregular at home so don't want to faff around with redelivery. This might just be a quirk of where the nearest depots of RM vs the couriers happen to be in my area, of course.
Plus, automatic cover up to £500 and you can purchase a higher limit at relatively low cost too. Filing a claim is a bit of a palaver, though (had to do it once and they took many months to resolve it. It was an open & shut case, too).