Be aware that 100 watt headlights are completely illegal in the UK, and if you fit them without adding relays you stand a very high chance of having a total headlight failure one night, as the wiring and switches will be seriously overloaded.
You would, as an acceptable minimum, need to rewire the lot, including the earth returns, with heavier wire, and fit FOUR relays and preferably FOUR fuses. You can use 2 fuses, left and right legally, but less than that, and certainly using one relay to control left and right lights, adds very serious risk and in most cases is illegal.
So either you MUST get into serious rewiring, and make a proper job of it (no cheap, nasty crimp tools for a start) or content yourself by fitting decent headlighs of the normal power rating, typically 55 watts. Those already recommended will make an improvement.
Just before someone makes the usual suggestion (they have been slow, perhaps they have learned something at last, which would be good), you can't legally fit either HID or most LED headlights. However, some of the lowest power LED lights (I think the limit is 1800 or 2000 lumens, but you will need to check the Construction & Use Regs), are probably allowable. You don't specify LED lights by power, a 100 watt LED headlight would suffer an immediate meltdown anyway. Now I think that LEDs are a very bad way to go, but only as they currently are, for reasons of future maintainability, limited life expectancy and cost. They can, if well designed, and E marked, provide good lighting, but LED technology is fast-moving and it is certain that you will be able to get a standard bulb long after today's LEDs are obsolete. They don't last nearly as long as typical solid state electronics, because the way they are presently designed is wrong, which causes the operating temperature to be high, resulting in light output degrading with time at a stupidly high rate, which is random between one LED and another. A properly designed LED headlight would not use one single LED cluster, running stupidly hot, but would actually consist of an array of tiny headlight units, each with a small LED, so the heat dissipation was not localised at one point, the root cause of the problem with present designs. I was playing with some LED stage lighting the other day, which works on that principle, and less than 30 watts was roughly equivalent to an old-fashioned 500 watt bulb. In the stage lighting case, there were serious limitations, because masks and barn doors could not give a well defined edge to the illuminated area, but a headlight does not need to be variable in that way, except between main and dip beams, so each elemeny of the array would be fixed, with its own little lens and reflector, and some would be switched off for dip. One day we may get properly engineered LED lights, not just yet apparently. The efficiency of LEDs is not going to increase significantly, as they are already close to the limit set by the laws of physics, but the life may improve.
