For tours like this, the real work will be done by a local "incoming" tour company. The company at the UK end just puts together your requirements, sends it off to Colombo, and comes back with a price, which they then mark up.
We went to Sri Lanka about a decade ago and got two prices. One was nearly double the other for essentially the same tour. So make sure you get two or three offers. We ended up with a company based in Germany owned by a German-Sri Lankan couple, and when we got there it turned out our driver knew the Sri Lankan lady in person.
You could also consider going straight to a local company — put inbound tours sri lanka into Google. Obviously check review sites before parting with your money, but these are the companies that Kuoni or Trailfinders will be dealing with anyway.
We loved our tour. We could have been in a small minibus, but because there was just the two of us we ended up in a nice saloon car with a very friendly driver. Only one topic (politics) was off limits.
A fun story: On the first day of our tour we stopped for lunch one day at a roadside buffet place that catered mostly to tourists. The food was local but "adapted" for Western tastes. The driver asked how we liked it, and we said "Fine, but it wasn't very spicy" (we love very spicy food). He thought for a moment and said "Would you like to eat what the drivers eat?". So the next day when we arrived somewhere for lunch, he went off and negotiated, and some "interesting" dishes were brought out from the kitchen just for us. We could see a couple of heads poking round a screen to see how the Europeans were getting on with the fish curry. It was great!