Hmm. That's gong to be hard. Not hard as in physically, the terrain is easy enough. Hard as in route planning. Your problem with a 100 mile point-to-point where the 2 points are 93 miles apart is that you don't have much wiggle room for deviations from a straight line. You are therefore forced onto the shortest possible route, which is naturally the main roads. This is tolerable at the northern end, but for the last half of the ride you have to dodge the M1, A1, M25, A14...the list is long. These sweep you into big towns and cities. Yuk. You need to plan a route where a few turns take you out into the sticks, which is why most century rides don't go much more than 30 miles or so from the start point
I've had a look through a couple of guidebooks and a road map. From Bourne there are a number of MUCH more pleasant rides, if you fancy a point-to-point then you still can. Ride to (say) somewhere in Northamptonshire, get a train home. This could take you past Rutland Water, through Oakham, all sorts of nice places. Alternatively you could be negotiating the A414 at Hemel. You choose.
3 routes that I came across that are do-able from Bourne were one that starts and finishes in Boston, which is easy but having worked in the area I don't recommend it unless you have an interest in cabbage fields. Better is one that starts at Melton Mowbray and explores the Vale of Belvoir, which you could join at Colsterworth, 10 miles west of Bourne, or another that explores Rutland, going through Oakham. You could drop into this from the top and then loop down towards Mkt Harborough. You could even combine bits of the two. I'm happy to share these routes sdo you can have a look on a road map.
Any and all of these would be a better choice than threading your way round the M25/M1 interchange when you are 90-odd miles in, knackered and just want it to stop.
If I lived in Watford and wanted to do a century nearby, I'd first drive to the countryside. If I didn't have access to a car or a train, then looking at the map there are the villages between Hemel and Amersham that take you to the north of Chesham. Minor roads to Tring, then over/under/across the A41. Head NW in an approximately straight line to Winslow. Explore the Chilterns, back via minor roads around Thame. But given the choice I'd start in Aylesbury.
My guidebook has century rides in the Chilterns and Oxfordshire, I'd be having a look at that and drawing on some maps.