MOT Wise, If it isn't a halogen bulb, it is a fail:- That being said, it's the highbeam - If you're dazzling people with anything, you're doing it wrong!
In practice, you've got a few upgrade options for the High beam.
1. Legal, but least impactful:- Upgrade bulbs (Nightbreaker Laser or similar) - These will give you a little extra brightness, but it'll still be "warm" light.
2. Illegal, slight improvement:- 100W Halogen bulbs. I haven't tried these in person, but I've seen some evidence that actual 100W (RallyMaster) will yield improvement; However - There are requirements for wiring and fuses so as to not torch the car.
3. Illegal, Easy to fit, instant on, and a big improvement (Though you may sacrifice some of the beam pattern) - LED Upgrade bulbs. They'll slot straight in, and be a big improvement. Make sure you get something with a proper heatsink - The "Foglight" LED bulbs are awful. The light will come on instantly, and be much better than Halogens - Though I find there is a slight dim spot at the very centre of vision, which is not ideal (That's on a 9G reflector)
4. Illegal, Absolute highest possible brightness, but a good beam pattern and colour:- HIDs (You may as well spring for 55W HID High beams; These will give you the absolute best light output - And good beam patterns. The downsides; The bulbs won't last long (That being said, I have had them last a winter), and there is an initial warmup time, of ~10 seconds before you're up to speed. This makes flashing less effective (That said, the initial strike tends to be incredibly bright), but it isn't an issue on a country lane - If you dip for oncoming traffic and switch back on, it is a "Hot restrike", and you'll be instantly back to 100% Brightness.
You're in luck - I've just purchased a luxmeter, and a new set of HID ballasts as I plan to switch my High beams to HID for a trip to the NC500 next week - I've got LEDs in at the moment, so I'll see if I can do some testing for you.
