20lpm from the cold tap is quite good. I was expecting, with an old Victorian pipe, that you would have poor flow. If you are planning to do away with the cold tank in the loft, you will need good flow with two bathrooms and (presumably) several people. I would be looking at a conventional or system boiler, with an unvented cylinder such as a Megaflo or other brand. It will give better hot water supply than a combi. Being simpler, the boiler will also have less to go wrong.
When the old lead pipe starts leaking, as it will, replace it with 25mm or, better, 32mm, right out to the pavement. It will give you better flow, which modern plumbing systems need. If you are doing much work on the house, do it before it starts leaking. You will need to trench across the drive or garden, which will upset you once you have landscaped or renewed it. You will also need to take up some floorboards in the hall, or continue the trench at the side of the house if you have access.
As you have lead pipes, ask your water co to test your drinking water for lead content. It is especially damaging to brain development in babies and children, and there may be a lead replacement subsidy if you run a new pipe. Ask for the test straight away, because it may take a long time to arrange.
Remember than any modern boiler will emit clouds of steam from the flue in cold weather (combis are worse for this), so position it where it will not blow past windows or billow round the front of the house. You can get flue diverter or extension kits if there is not a place for the boiler to avoid this, but they will not make your home more beautiful. Do not blow the steam at your neighbour. The boiler will need to be close to an internal drain such as a sink, otherwise the condensate pipe may run outside, where it will freeze in cold weather and your boiler will stio working. Please don't put it in the loft.