run the kitchen sink cold water tap into a bucket. Time it using the second hand of your watch. Calculate how many litres per minute it delivers.
This is the maximum amount of flow that you will get at any time from all the taps (hot and cold) in the house if you have a combi.
Filling a bath will use about 100 litres.
A combi-fed shower is usually good, but will the bath fill up in what you consider to be an acceptable time?
This is a very important question to ask if you are thinking of changing a tank-fed hw system to a combi.
If the flow is insufficient you can improve it by running a new water-main out to the street.
If there are likely to be two or more people running taps at the same time, the flow at each will be approximately halved.
having a boiler in a loft is rather tiresome, some people like to do it because they believe that the space gained will not get filled up with more junk stuff.
the point about gas engineers refusing to work in a loft if there is not a permanent secure means of access, good lighting and a safety rail is true. But you have the additional risk that since the loft is usually unheated, the boiler and its pipework will be at risk of freezing and bursting in cold weather.
You must also consider how to run the condensate pipe, preferably inside the house to an internal drain pipe. If it runs outside it will very likely freeze and block with ice in winter, and your boiler will then stop working. That happened a lot last year.