Hello. I am a solicitor with over 15 years experience. I will try to help you with this.
Have you told your landlord about these problems?
Yes still here. Have you told the agents and if so when?
You should put it in writing and demand that they fix the defective showers. If you have a written tenancy agreement then look up the clause that requires them to keep you home in good repair and refer to that. If you do not have a written agreement then there is still an obligation on the landlord to repair the showers.
What sort of tenancy do you have are you an assured shorthold tenant?
You have no right to withhold rent for this reason. If you are an AST and have been in the house for some time it may well be that you are a periodic tenant and the agent/landlord would be able to obtain possession on 2 month's notice. Withholding rent may trigger possession proceedings. You could threaten to bring proceedings seeking compensation for the disrepair and possibly seeking an order for repairs to be carried out. You will want this to be in the small claims court (to keep costs low and to avoid risk of having to pay the landlord's legal costs if you lose) and this will only be possible if the compensation sought is less than £1,000 and the cost of repair work is less than £1,000. If you do this there is a risk of possession proceedings being started. You are in a weak position I am afraid and your only practical remedy may be to leave.