Depends on your company policy.
I know our company insist on written applications and understanding of English before even getting through the first interview stage.
Main problem is due to eastern European immigration to the UK, if you cater for every language, you may come a cropper if the one person you employ who you haven't made an allowance for has an accident, it could be argued that the employer policy of accommodating the lack of English speakers by translating into their native language was missed and a failure of the company that will leave them liable.
So to prevent any confusion, making it a condition of employment by being able to speak the lingo will make your live easier, whilst giving you some degree of protection.
I appreciate if doesn't sound very PC, but there is a point to argue that from a health and safety view, safety in understanding instruction in English is a pre-requisite to working on site, the onus needs to be on the employee to prove he can speak, read and write English.
Also you can take solace in the fact that anyone coming to the UK to reside must speak English as is the governments migration policy.