There is a fair amount of misinformation about LED's for automotive usage.
So far the only light that CAN NOT be replaced by LED's is the main/high beam of your front lights. This is mainly due to the beam pattern produced by LED's. That being said ALL of the other lights in your vehical can be replaced with LED's. But I hear you ask 'Why should I change to LED's?' and the answer is that because LED's use less power than normal bulbs, so there is more power left over for things like your sound system or even for the performance coil and ignition system. More power = bigger/better spark = more horse power out.
The main thing to make sure of when using LED's is that firstly they are able to work at 12volts (as standard most LED's work at about 2.5volts) and secondly they are bright enough. I personally use LED's with a power output of either 1watt or 3watts (and believe me these are BRIGHT) and I can make a rear light for a motorcycle with only 3 x 3watt LED's and still have it completly legal and MOT'able.
When replacing indicator bulbs to LED you will notice that the flash rate increases hugely or on cars some of them try and tell you that you have a blown bulb. This is because the power requirement is so low, it is now around 0.7-1.4amps rather than 3.5-6amps, so you need to either use a new indicator relay (about £9 - £15) or ballast resistors (for 2 resistors from £3.25 to £11.98).
Some traders are using ceramic resistors which MUST have 1 per indicator changed, or for the metal clad resistor you only need 2, that is 1 each side (even though some dodgy information says you need 1 per indicator changed). The main advantage to using the metal clad resistors is that they get rid of the heat build up quicker, and as there is only a need for 2 they are easier to hide, I usually put them under the seat at the rear indicators as this is a easy to reach area.
When changing to LED's for your stop/tail light the MOT is a bit vague. But as long as your brake light is significantly brighter than you tail light, is not affected by the operation of other lights and emits a steady brightness, it should pass the MOT. There is NO way to measure the output of a brake/tail light (the official manual is online at UKMOT dot COM for clarification).
With some of the newer cars you have a 'Blown Bulb' warning light on the dash which lights up when you change to LED's, and the way to get rid of this is to use a resistor (wired in the same way as the rest).