Hey, that would be pretty cool to drive an ol' 122 around the old streets of London. Are you in central London, or the outskirts?

Here in the US we now have some replacement bulbs available which are a collection of red LEDs. The conversion is as simple as unscrewing the old-style and screwing in the new; no rewiring required. I just did it on mine and they are just as bright as the old, yet draw significanly less amperage. I'm not sure the exact figures, but if you look at christmas tree light strings, buying an LED string with the same number of ligts reduces your power usage by about a power of ten (aprox. 4.5-watts for a whole string of LEDs).

So... that might be a way to bring down your amperage enough not to pass that threshold of lighting of your dash-light.