If you want to do a proper job, spend a few bob at frost.co.uk or http://stores.ebay.co.uk/dinitroluk
and get some 3125 and 4941. Get a wire brush in a drill and take the old underseal back to good metal, and then remove as much surface rust as you can.
Dinitrol do a rust convertor, but its hugely expensive, you could use something like jenolite or granville, theres a hammerite branded kurust in halfrauds, et al, thats a green gloop, probably natural-eco-friendly-not-tested-on-polar-bears, but it certainly does **** all to rust!
The 3125 will bond onto any rust, but probably worth "converting" any surface rust.
The 4941 goes on top once the 3125 has dried and gives a rubbery extra layer.
Looking at the extent of the rust, I would suspect its coming from inside the sills! So in addition to doing the outside, get an extension nozzle like : Extension Hose (60cm) | Frost Auto Restoration Techniques
(some of the packs on ebay come with this) take the plastic sill covers off and give the insides of the sills from front to back and over the rear arches (via the boot) a good squirt of either 3125 or they also do some ML Cavity wax which is runnier and will seep into seams more than 3125.