Renting to friends: You would still need a proper AST contract (because they get one in law, so may as well have it written down so you both know the rules). You would need to supply an address in England/Wales for the service of documents, friends would have to pay a percentage of rent to the tax man (inconvenient) - unless you get exemption http://www.hmrc.gov.uk/cnr/nr_landlords.htm. You would still need a thorough inventory/condition report that would stand up in court. Deposits would not be lodged with a solicitor - you can use the free DPS at www.depositprotection.com
Renting through an agent - they will do the above, and the increased rent should cover their fees. This way you don't lose friends if the thing goes bad - and it can do HOWEVER agents are unregulated - you could become one tommorrow even with your current level of knowledge. So get one who is recommended, and it is often better to get one who is a member of a trade association like ARLA, though it is no guarantee.
I take it you are aware that you can't just tell the tenants that you want the house back? Make sure you know the processes and the timescales.
If you like the information/help provided - Gizza Job
(People under my age won't understand the significance of those last 2 words)