It sounds to me that the main sewer to your home is blocked up, if it is your own home and not council you will im afraid have to pay to unblock, if council they should do it free of charge.it may also be that other people living next door have the same problem if so it is a shared expense as the blockage may not be in your drain, you should ask them?if they have no blockage then it is your drain, the cheapest way is to hire some rods or if you can buy some about �20 do it yourself it is not a nice job to do, but a lot cheaper, that is what i had to do as it is a constant problem where i live due to a very bad bend in the road, as to your shower is the shower head scaled up unscrew and clean or it may have been a drop in water pressure if electric,or some internal component which i cant help you with. keep looking for more answers.