Most importantly, don'tuse any bike or component that you haven't thoroughly test-ridden before the event. And that doesn't mean once round the block, it means a two-hour test ride, minimum. Don't assume your various installations, adjustments, repairs and tinkerings are fine - road test!
Don't think you will be disgraced if you turn up with anything more modest than a new £2K bike. A decent sportive bike is something that you should be comfortable on all day long, yet is just responsive enough for you to imagine (in your better moments) that your'e riding Le Tour. Give prior attention to your contact points: saddle, shoes, handlebar tape and track mitts (short finger gloves). Here again, comfort is the top priority.
Tyres: You are strongly advised to use tyres having an anti-puncture layer extending to give some sidewall protection as well. Your tyres should also have a high TPI number (strands in the casing) and a minimum width of 23mm. On this course, 25mm is recommended and even 28mm is reasonable. Inflate to the recommended pressure with a track pump with gauge. Carry at least two spare inners - unless of course you've gone tubeless.
Cycle computer: it rather makes sense to set it to MILES and not kms for this event. Only mile distances are given on the route card and (as it happens) on UK road signs.
Gearing: Compact chainsets are desirable (any Triples still out there?). Though if you have a 42-tooth racing inner ring, it should be enough for the steepest hills in the HHH. If you can't adjust your gears perfectly, get a competent bike shop to do it for you. Also make sure the lateral range of your front changer is stopped off correctly so that it can't throw your chain off as you change from one chainring to the other.