Erm, how long is a piece of string? While you're trying to work it out (or waiting for possible replies from people with personal experience), here are a few quotes to guide you:
Quote 1:
"ARMA (the Association of Residential Managing Agents) estimates the average service charge bill in London at around £1,800 to £2,000 a year"
Source:
https://hoa.org.uk/advice/guides-for-homeowners/i-am-managing-2/should-i-extend-my-lease/service-charges-and-maintenance-companies-problems-with-your-leasehold-property/
Quote 2 (from 3 years ago):
"Buyers of apartments in new-build towers not only have to pay sky-high prices but also face soaring annual service charges that now average £2,777 a year, according to new research.
Some of the worst service charges are found in new apartment blocks lining the River Thames in London, where residents will have to pay nearly £7,000 a year on a two-bed flat"
Source:
https://www.theguardian.com/money/2016/mar/09/service-charges-soar-to-average-2777-per-year-for-new-build-flats
Quote 3 (from 2 years ago):
"A £7,600-plus service charge on my flat above a shop is bankrupting me"
Source:
https://www.theguardian.com/money/2017/sep/01/service-charge-flat-bankrupting-me-heinz-property
Quote 4:
"A new build property in Lambeth, South London, is charging leaseholders an eye-watering £7 per square foot service charge. This means if you bought an 800 square foot flat, you’d be looking at hefty £5,600 charges. You’ll usually pay less if you’re further away from the centre of London. For example, one property coming onto the market this year in Croydon, part of outer South London, carries a service charge of £1.55 per square foot, making annual service charges for an 800 square foot flat a more reasonable £1,240 a year"
Source:
https://www.directline.com/home-cover/property-service-charges