Whether a certain size tire will fit on the rim is one thing. Whether the fork and/or chain stay/seat tube clearance is sufficient to accommodate that size tire is something else.
I see Boardman bikes, but I don't see a 2018 model. If that is a model year, then you need to provide info on the model, since Boardman seems to make a full line of bikes (road/racing, hybrid/commuting, mountain). If you don't have specs on clearance for tires for your particular model (some manufacturers will give info on what size tires are appropriate for certain models, but many don't), then your best bet is to bring your bike to a bike shop and get their advice. Unless you can find someone in this forum with your bike or a very similar one, it will be very hard for any forumite to give you definitive answers. .
What brand and model of tires do you have on the bike now? How much clearance is there between the current tire and the fork (crown and blades)? How much clearance is there between the tire and the seat stays at the narrowest point, and between the tire and the seat tube? Given that info, you might get someone willing to give advice on how much bigger you might be able to go.
Also, if you're fitting fenders on your bike for your tour, be aware that will affect the clearance, and may force you to use a smaller tire.