Just my 2 cents worth here. Your headlight runs from 12 to approx 13.5vdc with a current draw of 5.4 to 4.8 amps respectfully
for the 65 watt high beam.
For a 100 watt bulb for the same voltage will draw 8.3 to 7.4 amps. This is quite a jump and you will have to change more than the wire. I would use a relay to control the headlight with heavier wire going directly to the battery with a ceramic H4 connector. Eastern Beaver (no affiliation) makes H4 Heavy Duty Single Headlight Relay Kits with Ceramic H4 Socket that will do the job. Here is the site: http://www.easternbeaver.com/Main/Wi...s/h4_kits.html
I will not comment on want is legal in the UK but I am pretty sure in Canada that it is not.