According to Google’s AI, a Sberbank branch in Vladivostok would give you a slightly better rate for Yuan today compared to USD or EUR. GBP should be avoided at all costs!
USD: 79.60/87.80 9.8% spread
EUR: 94.10/102.80 8.84% spread
CNY: 12.11/13.07 7.63% spread
GBP: 91.90/122.7 28.7% spread
You should also look at which of the foreign currencies are the cheapest for you to buy with your GBP. My understanding is that if you buy Yuan with GBP in China, you would get a better rate (lower commission/spread) than buying euros or dollars with GBP in the UK. (You should wait till you get to China to buy Yuan as you will get a much better rate than in the UK).
In this case, buying roubles with Yuan would win hands down, giving you a better rate both when you buy the Yuan with GBP in China and also a better rate when you buy the roubles with Yuan in Russia.