While 180 miles in a day is possible, frankly, it doesn't sound like much fun to most people. Sure, with sufficient training, you'll get some of the group up to the level of making that distance. You will also see a number drop out before the training even gets serious, once they understand the level of commitment and time needed to reach that goal. Hell, even 60 miles a day for a poorly prepared rider could be daunting.

What would the terrain look like along the proposed route?

For that distance, with the wide disparity in riding ability/duration, what would sound like fun is riding this over 2 or even 3 days, with time for stories over drinks and a good dinner each evening. If you can have SAG support to carry the luggage, all the better.

As a side note, is this an officially sponsored company event? Here in the States, if this were official, the Company could face an unexpected workers' compensation exposure. I don't know wht, if anything, this translates to in the UK, but it is something worth considering.

Gotta say, this sounds like a half-baked idea. It might morph into something more feasible, but it might just as readily fall apart.