depends what you brought and what it is. a cheap chineese bike may not come with a v5 or tax as it may need registering yourself. if its a proper bikesuch as a yamaha etc it should be otr (on the road) with full v5 and paperwork complete and tax
Your better off going japanese, chinese build quality an reliability for the most is shite!, think they know this so put the bikes out for sale with an attractive £ tag - most not knowing you have to for the most part do all the work to get it otr yourself, As for getting otr via a dealer,you don't have to get a bike that's not tailoured to your needs,mk. .
My new Honda came with RFL disc, the dealer doesn't supply the V5, it comes in the post about a week later. The bike comes ready to go, if you buy from a dealer. If you purchase online, it may well be crate-delivered and require some sort of build before being usable. Also, you'll most likely need to register it, and possibly get it inspected as well, and number plates made up.