Thought it was worth mentioning this one ... after getting stranded at Gatwick by a flat battery last year (probably my fault - the car had a central locking fault at the time), a colleague recommended one of these:
https://www.amazon.co.uk/DBPOWER-12000mAh-Portable-Emergency-Flashlight/dp/B01DLRGY8A/ref=sr_1_6?keywords=dbpower+battery+booster&qid=1554488342&s=gateway&sr=8-6I managed to find a nearly-new one on eBay for a bit less, and stashed it under my driver's seat. Fast forward six months to a trip away with some friends, flat battery in the morning (probably somebody left the boot lid ajar or similar) - bonnet up, clipped the booster onto the battery terminals, she started first time, problem solved (total delay maybe 90 seconds).
These things can allegedly jump start petrol engines up to 6 litres, so the GTi was no problem (quite a drop in percentage charge though - don't waste it on charging your phone etc.!)
For less than the price of a year's breakdown cover, I'd say every car should carry one. Three breakdowns in 13 years of driving, all of them were flat batteries. No more of those for me