I'd say the best option would be to "wire out" the SOX ballast
, replace the BC lampholder (about £1.50), and replace with an LED
replacement bulb (about £50 each if bought in bulk I'd imagine - seen them on WWW for £60 per single unit).
That way they keep all of the old SOX
lanterns. The SON
lanterns are easier to retrofit with just the lamp being changed and the ballast
All info is on the above site. I must say I haven't seen them in use, but surely it's a cheaper/more desireable option than total replacement. It's also very "green" because there aren't thousands of old luminaires in the dustbin!