Go to your local registry office by making appointment, complete paperwork with all her details, they publish details of your intention to marry asking, like at a marriage ceremony, if anyone has legal objection, if nobody objects then a couple (ish) of weeks later you get your certificate.
To get married in Ukraine you firstly need to go to your local Registrar in UK and get a No Impediment to Marriage Certificate. You will then need to get it Apostilled - http://www.fco.gov.uk/legalisation/
When you have done this you need to get a Notary in the Ukraine to translate into Ukrainian (not Russian)and certify a copy of your passport and the No Impediment to Marriage Certificate (Kateryna Taylor can help you with this) and then take all the documents to the Registry Office in the Ukraine.
Kateryna Taylor has just helped us to apply for a guest visa and, hopefully, Iryna will be arriving in 2 weeks for 18 days....!!!
I have made an appointment for the Certificate of No Imnpediment (CNI) aka 'Form 36'.
My passport has been translated/certified.
I must get my Birth Certificate & CNI certified/legally stamped...
Then translated into Ukrainian and certified by Ministry of Justice, Ukraine....
It's not all flowers and dancing when things get serious with a FSU lady!!!