You need to apply to the Gambing Commission. Try here first:

apply for one

it is a very costly license unless your making a ton its not worth it

ur better off promoting existing places

Sounds like you will need a remote licence, here’s what the gambling commission has to say about it:

Operators can for the first time provide remote gambling facilities using British based equipment.

Remote gambling is defined in the Act as gambling in which persons participate by the use of remote communication, including the internet, telephone, television, radio or any other kind of electronic or other technology for facilitating communication.

Under the Gambling Act 2005, remote gambling operators need an operating licence from the Gambling Commission, and the majority of senior management need personal licences. The provision of facilities for gambling without obtaining an operating licence will be an offence if at least one key piece of remote gambling equipment is situated in Great Britain (as defined in the Act), irrespective of whether or not the facilities are provided for use wholly or partly in the UK.

I think you will also need a personal licence.