I would advise on a start fund of £50k…out of this would come the truck, you won`t get much sense out of a bank regarding a loan at the start…fair weather friends, beware 
£50k less a good “used” tractor, leaves a fighting fund to cope with everything that WILL come your way in the first few months.
Also, with this amount of “cash”, the licensing authority will take you seriously, any less and its questions, proof + hassle 
You can get a very good Volvo FH13 for less than £20k, pretty much bullet proof and something decent to drive everyday 
Have you factored in the insurance element ?, this will be circa £5k for a start up venture.
I`d rent a trailer for the time being, just to suss out the “job”, a lot of potential customers will have you pulling their trailers or, you might get lucky, but the customer needs a 4.5m …then you have 4.0m sitting in the yard
It aint easy, thats for sure, and no doubt someone will disagree with my advice, i would just take in what everybody says, AND listen very carefully to your accountant