As an essential condition for entry to Bahrain territory, it is a passport, valid minimally for six months since entry date, and, of course, a valid entry visa.
We wish to recommend you to check, before you leave, at Bahrain Embassy, whether the conditions for entry and stay in Bahrain did not changed in the meantime. The request for visa granting could be presented at Consular Section of any Bahrain Embassy abroad, or you can apply for a visa in some of Bahrain Five-Star hotels (Holiday Inn, Diplomat, Regency, Al Khaleey, Hilton, Meridian, Sheraton).
Consular Section of Bahrain Embassy in London is as follows:
e-mail address: firstname.lastname@example.org
Visa application form would be accompanied with two photos and invitation letter from Bahrain partner.
Bahrain Offices issue these three sorts of entry visas:
a) tourist visa, valid for 14 days
b) diplomatic, valid for six months (one year for Ambassadors)
c) special visa, valid for three months
Visa prolongation is possible to be effected for next fortnight.
Police registration for visitors is not obligatory.
Kingdom of Bahrain
Information and warnings on travelling abroad is to be found HERE.