Early warnings you can check yourself between services
You do not need a workshop to keep an eye on a battery; you need two minutes a month and a little discipline. The cheapest tool is a plug-in voltmeter for the 12V socket, sold in every accessory shop for the price of a coffee round. Read it before the first start of the day: 12.5-12.6V is a full, healthy battery, 12.2V means it is living half-charged, and anything starting with 11 means the battery is either failing or something is draining it overnight. Take the reading at the same time of day for a fair comparison, because a battery straight after a drive always looks better than it is.
None of this replaces a proper conductance test, but it changes the timing of everything. Owners who spot the 12.2V trend in March replace on their own schedule, at a normal price, from a shortlist of vendors. Owners who wait for the no-start discover it in an August basement and take whoever can come fastest. The whole economics of battery replacement in this country favours the person who moves three weeks early.