Shared water supply with combi boiler but shower hot water pressure extremely low
check if the shower head is clogged with limestone
check if the mixer is clogged (usually only hot supply gets clogged)
check that the combi isn't clogged: get a bucket, get a watch, weight it empty, go to another tap (served by the same boiler), open hot water at maximum heat and maximum flow, fill the bucket for exactly 60 seconds. If the bucket gained less than 6 kilos, the combi heat exchanger got clogged and you have to call the assistance. You'll have to pay about £150 to clean it up.
In winter hot water flow is usually reduced due to reduced mains temperature. (But usually it shouldn't be too noticeable)


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Little late: I didn't looked at the time the question was made