The Maplin solution would be a lot cheaper.
An 0.22 ohm resistor in series with the lamp would give you 0.9v drop and an 0.47 ohm resistor 1.8v. (approx assuming 12v)
A 10w wirewound will be adequate, your looking at approx 4w for the .22 ohm and 8w for the .47 ohm.
They will dissipate heat so keep them away from other wiring, putting them in the earth return side of the lamp will protect the high beam filaments as well.