From my understanding, the emergency lights you plan to install are normally off and will only light up upon a power failure. Also, these emergency lights will be wired separately from the regular 220V lines. If that's the case, you don't really need a UPS.

What you need is a trickle charger and a suitable-sized 12V battery (deep-cycle is best).

The lights can be 12V MR16 LEDs. These lamps can operate in either AC or DC and can be directly powered by the 12V battery upon a power failure.

An automatic switch can also be DIYed using a small power supply and a suitable-sized relay (in a normally-closed configuration), so while there's utility power from the outlet, the relay is energized, preventing the emergency lights from lighting up. But during a blackout, the relay loses power and goes back to its normally-closed configuration and completes the circuit to supply current to the emergency lights.

Why not a UPS?

1. The output side is always ON whenever the UPS is powered up. In practical terms, even without a power failure, the emergency lamps are lit. Designing an automatic circuit requires a lot of caution as you're dealing with 220V AC.
2. The process of converting from battery (DC) to 220V AC produces heat and parasitic loss, eating up battery capacity when it matters the most.