upgrading circuit box and had shower fitted - do I have to upgrade all circuits?

Hi, Your description is a little confusing, but no, you should not be changing all of your fuses to 40 Amps. The fuses are there to protect the cable and should be rated according to the size of the cable, the method of installation and the characteristics of the supply. This really isn't something you want to be doing yourself. It requires, by law, a qualified electrician to complete the work, and even if you're not worried about the law don't forget you'll be standing in a pool of water while you're using this electric shower, so any faults could be fatal. I'm probably not in your area, so this isn't an attempt to get the job. But, for your own safety and peace of mind, please use a qualified electrician to do this work. You know where to find one after all! All the best, Ian

You can't just change the circuit breakers in your fuse board they are rated based on the size of cable installed, When installing a new shower circuit this should be notified (it is illegal not to) your best to get an electrician around to check all is ok.

No each circuit will be fused mainly according to cable size and installation method. Altering circuit protection(fuse ratings) is something that should always be left to the professionals.

No. the size of the circuit protective device (CPD) is calculated based on the cable size and load current so you can't just go replacing fuses or circuit breakers! To do so without working out the loading on the cable and if it meets the required design and operating parameters specified within BS7671:2008 would be suicidal and could well result in a fire.