Not allowed to fit a pump.

You need to get the incoming mains pressure and flow rate tested.

Could be 1bar lower in the new shower.

Good plumbers merchants sell pressure guages.  The flow rate you get as a result of that pressure will depend on a lot of things including..

The diameter and length of the pipe from the road to your house.
The diameter and length of pipes within the house
Restrictions caused by scale, valves, bends and joints
The power rating of the combi boiler (determines how much water it can heat per min).

You can measure flow rate with a bucket of known volume and a watch with a second hand. Convert it to Liters per min.

Going upwards you loose 0.1bar per meter. A floor is around 3m so the pressure one floor up would be about 0.3 bar lower.  May not be a problem if you start with 2-3 bar.  

What might be more of a problem is running both showers at the same time. You might find the flow rate drops then. What happens if you run the existing shower and turn on another hot tap or two ?

The pressure guage on the combi probably measures the pressure in the central heating not the dhw.

Measure the pressure at a cold tap on the ground floor. If you have 3 bar there then on the first floor up (approx 3m higher) it will probably be around 2.7 bar. On the second floor around 2.4 bar etc.

The 14.03 L/min figure is the important one. There will normally be a footnote that says something like "with a 35C uplift" meaning that it can raise 14L/min by 35 centigrade. If the water comes in at 10C you get 14L/min at 45C out.  

The amount of water you need depends to some extent on the shower head and the water temperature (eg how much cold is added). Water saving shower heads draw around 7L/min I believe but if given the choice many people prefer higher flow rates around 10-12L/min. If you plan to install an old fashioned victorian rose or modern rain shower perhaps think again.

If you find it's not delivering enough water one option might be to add a  stored hot water tank. This would be plumbed into the central heating heating side of the boiler via a diverter valve. Perhaps leaving one shower on the regular DHW output. Before going down that route you would need to be very sure of the cause of the low flow rate (eg is it the boiler, incoming main or internal pipework).

A Glow worm compact 100p isn't the most efficient boiler in the world (71% & F grade according to google). If you were feeling rich or the boiler is unreliable you could consider upgrading the whole system with a new system boiler feeding a megaflow or similar mains pressure stored hot water system. These are generally better than combi systems when you have more than one shower. Perhaps wait and see.

PS. I guess you could try experimenting by running a hose from the kitchen tap up the stairs and out of an upstairs window to simulate the shower upstairs. See what sort of flow rate you get with that and the existing shower going. Don't flood the place though!