The single best thing you can do to preserve the gas in your aircon is to use it regularly, even in winter. If you make sure you use it at least once a week, this circulates the PAG oil throughout the system, wetting the seals and o-rings and minimising coolant leakage.
The only problem with top up kits is that you have no idea how much oil is in there. Depending where the system is leaking, you could be losing predominantly gas, or you could be losing predominantly oil. You are adding more oil with a top up when the system doesn't need it, reducing cooling ability, or be left with too little, which could spell disaster for the compressor.
If you don't know what the situation is exactly, you would be better off with a full vacuum and regas, so that the oil can be recovered and weighed, and the correct amount of oil and gas replaced. KwikFit and similar garages often do offers at around £25-£30, with no charge at all if there's no improvement. I wouldn't normally recommend places like KwikFit for anything, but the automatic regas stations they use are virtually idiot proof.