look at the website of your water company. Search for Lead Pipe Replacement. See if they have a subsidy. Also ask them to take a water sample from your kitchen tap to test it for lead content. Get the sample done before renewing or replacing the pipes. Some water companies are more helpful than others. Until they have tested the sample they won't help, and it might take weeks. If you have a baby in the house mention you are concerned about FF.
If you qualify, they will usually replace their bit of lead pipe (under the pavement and up to the boundary) at the same time that you replace your pipe (from the boundary into the house). This is very much worth doing, otherwise you will have to try to connnect a 25mm (or, better, 32mm) plastic pipe to a half-inch or 3/8" head pipe with obsolete threads on the stopcock, which is very hard and unsatisfactory. (I can vouch for that....)
If you don't already have a water meter, they might agree to dig up the pavement and fit one free, and connect your new pipe to it provided it is dug and ready. The old pipe can be used until the moment of switchover, so you need only be without water for half an hour or so.
If the old pipe is leaking, they will probably also have a scheme for subsidising its replacement. It might be a couple of hundred pounds or so. A water meter will show up any leakage because the bubble will never stop turning.
They will have a list of approved contractors, but you or a plumber or builder can dig a trench and lay a new pipe fairly easily (the plastic pipe is reasonably flexible) but they will insist on inspecting it before the trench is filled in to verify that the materials are correct and it is to the correct 30" depth to prevent frost, gardening or traffic damage.
Plumbers tend not to like heavy work like digging trenches, but a builder or labourer will do it. If you have a garden or gravel between the pavement and the house, and wooden, not concrete, floors it will be easier than if not.
Best to get a plumber to do the connections and pipework inside the house. You will need a new larger stopcock and 22mm copper pipe inside, otherwise the flow will be contricted.
Unless you have a lot of concrete, it should cost some hundreds, but less than a thousand. I did one in an Edwardian house a few years ago and I think it took a couple of days. Pipe and valves were less than a hundred. The new larger pipes are quieter, as well as giving far better flow, and if you use 22mm in the house, the hot water flow will be unaffected by running cold taps (which is a common problem with combis). The new flow should be ample to run a Megaflo should you so desire (I would).