The weather will be fine. It's not going to be a 2016 or 2007 etc, it'll be mostly dry with a bit of rain and potentially some mud over the weekend. Perfectly manageable and not enough to spoil the fun.
Sorry that's what meant. If you can buy it on the day for general parking I see no reason why you can't do the same for WV if you are staying there. Probably 5 to 10 quid more than if you buy in advance.