Hello @adam18 - welcome to our community and thank you for your question.
1) In accordance with the so-called Dublin III convention you are only allowed to have one asylum application. If you apply for asylum in Italy, Italian authorities will check back with the European database and find out that you already had a procedure in the UK. That means that you will be transfered back to Great Britain since they are responsible for your case.
2) Every country has its own rules about marriage, but if you fulfill all the requirements and if you can provide all the required documents it should be possible. Get in touch with the Italian registry office that you want to get married at in order to find out what you have to do exactly.