In order to get a work visa in Hong Kong, you must be sponsored by your full-time employer. There are legal requirements for this - for example, you must have at least completed a Bachelor degree - a TESOL certificate is not enough on it's own (even CELTA).
Check out the HK immigration website
Untitled there may be special arrangements for British citizens.
Hong Kong has a 12 month work/holiday scheme, but it is only available to certain nationalities.
Your best bet is to come to Hong Kong and make connections on the ground. You may even be able to arrange sponsorship with an employer if you do not have a university degree, but have lots of experience (I repeat, MAY...)
Unless you are highly qualified, I doubt that you will have much chance getting hired from the UK. There are some kindergarten groups that will hire teachers in their home countries - from what I have heard, the pay is low, the conditions are rough and the contracts are hard to break.
Higher end international schools expect teachers to have qualifications allowing them to teach in their home country and experience at teaching in schools in either the UK, USA, Canada, NZ or Australia. Often they require a teacher who has also had international school experience (in HK or elsewhere) and sometimes IB experience. As an ESL teacher, it will be difficult to get a job at one of the higher end schools.
You may like to look at
EMB - Native-speaking English Teacher (NET) Scheme