" very lukewarm trickle of water"
is it an electric shower that runs off the water main pressure?
When did it last work correctly?
If it worked better in summer, and has got colder recently, one of the causes will be that the incoming water supply is now much colder. Electric showers are very weedy little things, and only have the ability to raise a certain amount of water by a certain number of degrees. If the incoming water is now 10 degrees colder than it was in summer, so will be the outgoing water.
Electric showers might be 7kW or so. A typical modern gas combi boiler will be about 30kW, so you can see it can deliver about four times as much hot water.
Do you have a hot water cylinder, or a combi?
In terms of faults, if your electric shower has high/med/low settings, verify that it delivers heated water at each setting. Most electric showers have two heating elements, and they switch on the small, the large, or both elements depending on your setting.
If you buy a different, higher-power electric shower, it will not be much more powerful, and it may need a new supply cable and changes in the Consumer Unit which must be done by a qualified electrician who is a member of a Competent Person scheme.
Electric showers are fundamentally not very good.