13lpm is a bit meagre with a combi. 20lpm is OK.
A poor incoming flow means that showers are likely to run hot and cold if water is used elsewhere in the house at the same time. 12lpm is OK for a shower, but a bath holds about 100 litres so will take a while to fill.
It will do no harm to have a slightly more powerful boiler. Within the same range the prices are not vastly different for about 12kW/18kW/24kW/35kW output. But you need one that can modulate its power down well so it does not keep turning on and off at low loads. You can estimate the heating load by measuring your existing radiators and deciding if they are big enough to keep your house warm in winter, or if you want bigger ones. It will probably be less than the hot water load, if you have a combi.
There's a chance you have a fashionable Italian kitchen tap that constricts the flow (traditional British Pillar taps give the best flow) but an 80-y-o house is likely to have a small incoming waterpipe, probably lead and a bit squashed. For better water flow, you could lay a new plastic pipe all the way from the boiler to the meter or stopcock in the pavement. It is no complicated work, but it does involve digging a trench, which is rather wearisome if you have a concrete frontage.
Ask your water co to test your drinking water for lead content. There might be a subsidy or free reconnection in a lead replacement scheme. It will be inconspicuously hidden on their website.
If you had better flow, you could get either a more bountiful combi, or an unvented cylinder, which gives unsurpassed hot water.
Ask your plumber to check that your stopcock(s) and any service valves are big enough.
This sort are rather constricting and you can get better ones that don't leak so often.