So don't have the budget to do a lux design but I've been going on power, output, colour temp and CRI. This is mostly for corridor lighting. It is for the town hall, and it listed but that that won't prove an issue in this regard as all the new fixtures are similar and within the footprint of the older lights.
We have a separate emergency lighting but I may go for the emergency options anyways given the recent fire. I'll be doing the installation myself as no modifications are required. Besides there's not budget atm and I'll be footing the bill til we raise some funds but at the moment we're down 3 fixtures.
You might need NICEIC minor works installation certificates for the replacement but IANAElectrician. If you do a like for like emergency lighting replacement that's just an electrical issue. If you redesign the emergency lighting I think you will also need an Emergency Lighting Installation Completion Certificate, plus all the usual 3hr tests etc.
You can estimate the lux yourself. Lux is lumens per square metre so you should be able to work up a comparative estimate using the lumen output data for the old and new lamps.
https://en.wikipedia.org/wiki/LuxSome LED tubes have significantly lower lumen output that the old fluoroescents, although their lumens/Watt efficacy is far higher. If you're switching from 8' to 6' tubes on the same centre point you could end up with dimmer patches in the corridor but you might get round that by switching from single to twin battens for example, or the change in lux at floor level may be relatively un-noticable depending on ceiling height.
Consider regulations around the safe disposal of old fluorescent tubes. Your local authority should be able to advise on the correct recycling route. Good luck.