Yes, I rarely go direct for anything if there is discounts to be had with 3rd parties. You can transfer your number no problem. You can just text your operator for a PAK code, enter it into the website then ordering and your number will move a few working days after your contract is activated. To move your number you need to move networks but I think 3rd parties will sell you ‘upgrades’ at the same price if you are already a customer of the network.
If you want the lowest TCO consider paying more upfront. The more you pay upfront the less it tends to cost over the two years. It’s normally cheaper to get a 2 year contract from a 3rd party (while paying a chunk upfront) than buying the phone out right and setting a sim only. The most expensive is normally to go direct for a contract.
Consider using a price comparison site like uswitch, they tend to have even lower prices than going direct to a 3rd party even after Quidco/Topcashback.