Could you tell me why British electric locomotives(for example class 86,87 or 90)have only one pantograph?The continental locomotives(like Taurus class), Japanese or even American(GG1)have always two pantographs.
with best  wishes to all
Roman

Most older electrics were built with two pantographs and the old Woodhead DC electrix regularly used them both together. AC electric Classes AL1-5 (TOPS 81-5) were built with two, but only ever used one at a time, partly because at the higher line speeds 90+ mph, the vibrations from a leading pantograph would disturb the second one. At one time the practice was to use the trailing pantograph on a loco, but there must have been little difference in performance as by the time the Class AL6 (86) was built in 1965 (just 6 years after the AL1/81) it was thought unnecessary as if a pantograph broke it tended to bring down the overhead anyway, so a rescue loco would need to be sent to tow the dead electric anyway. I believe it was also said that stowed pantographs tended to suffer from wear from being rattled about.
Interestingly the Class 91s are now being fitted with a second pan arm.

The rear pantograph was used as in the event of pantograph failure the bits would sail over the cab out of the way while a lead pantograph failure was likely to result in fireworks on the roof or damage to the trailing pantograph.

Another consideration is that the UK has only one overhead system (discounting the channel tunnel height difference) so only need to cope with one wiring setup. Eurostar on the other hand does have double pantographs.

I believe the reason to use two pantographs at once on the Woodhead system was that at the lower voltage it used much more current. 

The second pantographs were removed from classes 81-85 to make room for air reservoirs when air brakes were fitted, but classes 86 and 87 kept this space clear so had space to fit a second pantograph.  A class 91 has experimentally been fitted with a double pantograph, and the Pendolino units also have two and normally raise only the one further from the front. 

An exception to the rule is 91 114, which has recently been fitted with dual pantographs. The second one was fitted as a reserve, in the event of a pan failure, I believe.