When I left, the turnover of the UK office had exceeded 50% in the previous 12 months.
Very American-centric, which means that some of the processes won't work in an international culture.
The benefits are the minimum legal requirements in the UK, and it takes ages to get them (though this had improved when I left)
Salaries are way below market rates for London, and raises are slightly below inflation, so you'll end up worse off financially than when you joined. No bonuses in the team I was in.
Outside of sales, don't hope for good commission on upsells or cross sells.
All the development is driven by processes, with strict SLAs that the development teams adhere to the letter. Hopeless in case of exceptional situations.
Some teams are completely overworked, and people quit on a weekly basis so all the time and effort spent in training the new hires is quickly lost,etc
Don't expect a good raise if you are internally promoted.
Location is ideal if you love looking at the traffic on the M4 :-)