well in that case you could just start with a BSO and replace it once you cross the English Channel! If the OP has £500 budget this implies limited funds to fix the bike en-route; so my point was not 'bollocks' - which was rather unnecessary use of language really.
Someone on one of the forums has cycled all over Europe with a £99 BSO without problems. And fixing it only costs a lot when you are in Europe or the USA (although I've had lots of running repairs done for free when touring) In the rest of the world bike fixing is pretty cheap because its an everyday form of transport and people don't earn a lot. As long as it has basic level Shimano gears, proper rather than plastic hub and BB bearings and metal rather than plastic brake levers and calipers it should last fine especially if you tighten up the wheel tension a bit which is usually a weak point of cheap bikes. And part of the adventure of touring is the challenges it throws up.