I have just carried out a six monthly three hour test on our EML, we have one room  approx. (26x26ft) where two walls are 50% large windows, the two ceiling lights have failed test, one fire escape door has a luminaire above it, there are double doors with viewing panels in another direction that does not have a luminaire above it but leads to other escape routes that could be used.

The building is occupied from 8.00am until 5.00pm Monday to Thursday then by a cleaner until 7.00pm and on a Friday from 8.00am until 3.00pm.

Is it necessary that the two failed lights are replaced as batteries are no longer available?

The times of occupation you stated then it is likely that EL is required because in winter it will be dark by 4.00 pm and assuming the EL system was installed correctly which only could be checked by physical inspection, consequently any defective units needs repairing or replacing. 

You only have to do a 3 hour test annually, there is no 6 monthly test. Yes it is likely you need to replace the fittings , but the cost is negligible so it's hardly capital expenditure: https://www.safelincs.co.uk/eden-led-emergency-bulkhead-light/