Hi, we have let out our property in London while we are in the US temporarily. We also use an agent to manage the property - both in terms of rent collection and general maintenance.
I'd get landlords' insurance to cover non-payment of rent and damage, and check that your buildings insurance is ok with tenants being in your property.
You'll need to register with HMRC as non-resident landlords so as not to pay tax on rental income below a certain level.
A gas safety certificate is mandatory, an electrical safety certificate is not (as someone incorrectly wrote above). However, you are still ultimately responsible for the safety of the property so I'd get a local electrician to carry out an electrical safety check every three years just to be on the safe side (3 years based on a newly rewired property).
Inform the council that the property is tenanted so you're not billed for council tax. In April, our local authority in the UK changed the rules and so we are still liable for a portion of the council tax even if the property is empty.
Have enough in the bank in the UK to cover a couple of months' rent, plus a major repair, eg a new boiler.
Is your property mortgaged? If so, do you have consent to let from the mortgage provider? If not, you'll be invalidating any mortgage agreement and usually insurance cover too. We pay £95/yr for a consent to let document from our bank.
You'll need to out the deposit the tenants give you into the DPS - this is a legal requirement.
We have a contract with our window cleaner who goes to our property every six weeks. We also gave our agent a list of our preferred contractors - plumber, electrician etc - as they all know us and the property and we trust them to do a good job. Our tenants know that they can contact us with issues, should the agent not be helpful (not happened so far but you never know).
Good luck with it - check out the Landlords' Association website for more details.