Atlassian themselves do not do this sort of thing, they use their expert partners to do it. There are many of them with a presence in London, and I'd suggest reaching out to one of them.
https://www.atlassian.com/resources/experts allows you to search by country - if you select the UK, you'll get a good list, I don't think there's any UK based expert who is not able to cover London easily - most of them have London based staff.
In the interests of being open and honest, I should point out that I've been working for one of the Experts as a consultant for the last 18 months, but you should probably pick out the ones who may be the best for you, not just go with the first one who offers any demos.
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