i am a UK citizen travelling to Shenzhen next week, how can i get a visa within a few days?

If you are in UK, you can apply for an entry permit at the CVASC office by using express service, which may take two or three working days. Or you can fly to HK first, and then take a ferry or train to Shenzhen and apply for it on arrival at Shenzhen. Note that the VOA is only applicable to Shenzhen.