If your target audience is in the UK, then a .co.uk domain will help you rank in the UK search results. As for disadvantages, I cannot think of any. Cryo.
The question is what is the advantage of registering a .co.uk extension. You look more credible if targetting the UK market, and can be found in the local results.
its more aimed at a uk audiance this can have problems if you have information on the site that you want to be available to a worldwide audiance
as u got domain name like a.co.uk it will help u to targeting the regional poeple. For exp if u selling some thing only for UK people than .co.uk domain will halp you to get good SEPR.
.co.uk would cater well to the UK market only, but if your target market is global and you or your business is not based in UK, registering a .co.uk won't be an advantage.