Yes youre right, they are a cheaper option but thats not to say the others arent good value for the whole lot, thes bikes are getting on for 30 years old now and many of the reflectors have seen better days, they bubble and then start flaking. and also those bulbs you mention are dearer £12.20, versus £7.99
If you go for them, I think this will be the one for you
American Pre-Focus Halogen replacement bulbs are now only available in 12V 50/50W & 6V 35/35W. The bulb these replace is shown as illustration 17 in the diagram of standard bulbs. £12.20 ea.
Click here for the diagrams of standard bulbs