Most houses in the UK had lead gas pipes. Lead was fitted as choice in most until WW2 and after. Iron started to replace lead befoire WW2 and after took off after. In some parts of the country gas pipes are still in iron. In many new houses iron is still the choice. In other parts of the country copper has been the norm in gas since WW2 , and still is and should be too.
Natural gas conversion crews never removed lead. In the 1970s lead was still very common, gas tight and working well.
In the Manchester area the area gas board was installing lead gas pipes for gas cooker and fores runs up until the late 1970s and probably beyond. I know friends in Chester who in 1978 had lead pipe delivered for a gas fire run and told them to take it away and fit copper. They did without question and all the same price. The fitters would feed the coiled lead through under the floor boards. One old trick was to open two floor boards at each end of the run, then send a car down with a string. It always came up at the other hole in the boards with the string, then they could pull through the coiled lead pipe.
There is nothing wrong with using an existing 3/4" lead gas pipe for a gas boiler. If it is gas tight then use it. The problem is that not many have the skills today to make a lead joint, that is why they rip it out. I have come across 100 year old lead pipe in walls that has been near perfect. The only problems with the pipe was where they rammed the iron lead hooks into the wall. These would sometime crimp the lead.
I once came across a house that still had gas lights in the 1970s, with no electricity in the house. Two old dears who still had a dolly tub. I have fitted gas lights, made by Veritas, they were popular after the power cuts in the 1970s. The Tower hotel at Tower Bridge had them fitted in certain areas. I think they are still there.