Halifax is about the centre dot of the UK (West Yorkshire and is just a stones throw from Bradford, Leeds, Wakefield, Huddersfield area.)
There is no legal issue with requesting a female employee only if you target a specific customer or customer base as a company, this could be in areas like hostels for abused women, female psychiatric wards etc where a male presence could be harmful or even just personel customer preference so don't worry about specifying you require a female Electrician. Where the issue arises is if you advertise a position for an Electrician for the general purpose electrical work where gender wouldn't be classed as an issue then you can breach laws regarding discrimination if you decline male interest purely on gender.
All you need to do when advertising for the position is express that due a clients preference or you target workbase you have a position available for a female electrician. This is fine as other companies do advertise like this like Taxi firms who have customers who prefer female drivers etc.. but setting up a company of all female staff is when things become tricky if your doing what is regarded as work that is gender friendly.
Also be careful that there is no discount based on gender, Sheila's Wheels car insurance were found in breach of EU discrimination laws when they gave a better insurance deal to females based purely on gender although I doubt this will effect your position here as I assume you will be charging the same fee to customers regardless of there request for a specific gender.
Good luck on you search, as an industry, we do have a shortage of females and there is a preferencial customer base out there for anyone that can offer a female Electrician as an option.