In reply to KevinD: one large project? Talk to rspb about doing something on the eastern moors, but unless you've got deep pockets, the logistics of getting all those people together on one site might be harder than you think. If you'd rather just deal with one coordinator on the other side, this might be easiest.
If you want local projects, the wildlife trusts would be your best bet. Identify nature reserves near your bases and talk to them about appropriate local projects - hopefully something more imaginative/ longer lasting than clean ups or litter picking, mind. The advantage of this is its much less hassle for your people as its close to home. The big downside is that it will probably need more organisation and you may have to liaise with multiple separate bodies, though the Trusts could probably help with that. If you want putting in touch with someone who might be able to help with this, pm me with more details and I'll pass it on.
How tight is your schedule. It's easier to find jobs for big groups later in the year, but often corporates aren't so keen to work in colder weather.
Whatever you decide, good luck with it.