As you mention a lockup I take it you have no electricity available to run a charger
Do you have any scope for parking it outside your house (or anyone elses) with an extension lead on the days you use it?
It would be worth you buying some jump leads and keeping thenm in the car to make it easy for any kind person to help you. Get the longer, heavy duty ones.
You can get a portable jump-start device that you could charge up at home and take with you.
A newish battery will hold its charge better than one that is 5yrs+ old.
Other suggestions: Try not to use battery-draining devices like heated hear window, fast fan, electric windows, headlamps except when the engine is running fairly fast (i.e. not at tickover). Run the engine at fast idle for a few minutes with no accessories switched on at the beginning and end of each journey to help charge the batt. Make sure there is no current-draining thing on while it is parked (interior or boot lamp; alarm are common). Verify that your rear foglamp is not switched on (except in fog or falling snow).
If yours is an oldish car with a carburettor, or it is possible in some way to adjust tickover speed, set it fairly fast.
I believe a 20-minute run is usually considerdd enough to keep a (not flat) battery topped up. But if you are stuck in traffic jams with the engine at tickover with headlamps, fan, HRW and wipers all going, this will flatten the battery as you are using more energy than the alternator is delivering.