I would wrap the tripod in bubble wrap to protect the legs and head from impact damage and send it in a box rather than a bag. Not only will a box provide better protection but you will be able to demonstrate careless handling/damage in transit easier should it arrive damaged. Yeah, I know it shouldn't happen but it does. Most tripods are too long for Royal Mail but any of the national parcel carriers should be fine - I always use Parcel Force but no matter which one you go for somebody will tell you a horror story! Make sure the carrier's insurance covers the value adequately and use a signed for service.
Bubble wrap can be bought in almost any office supplies shop (e.g. Staples) or from places like The Range and HobbyCraft. You might even pop into your local high street stores and see if they have any they're about to throw out.
Hope that helps