The aircraft required 550m from the 26 end but as runway 08 was in use they needed 700m as Luton does not have a full set of approach lights on 08 due to the road and railway line.
This is correct ~ the JAR-OPS minimum for Cat. 1 onto 08 is in fact 750 metres because of the reduced approach lighting (only three cross-bars) on RWY 08 owing to the topography. For RWY 26 which has a full-length system (well, almost...) having five cross-bars, JAR-OPS stipulates 600 metres.
Undoubtedly, if the visibility falls below 700 metres and/or there's a cloud ceiling at or below 200 feet there will be significant distruption to arrivals (in a visibility below 350 metres, also for departures), whilst Luton's lighting can only meet JAR-OPS Cat. 1 requirements although the ILS will be radiating to Cat. 3 (whenever the Runway is open) throughout the period of re-surfacing works.
Still, this has happended elsewhere during runway re-surfacing (e.g. Birmingham, Stansted) and there are plenty of airports that remain either Cat. 1 (e.g. Coventry, Exeter, Bournemouth), uncategorised (e.g. Southampton, London/City) or totally non-precision (Gatwick 26R/08L).