Your visa can only be issued in the name on your passport - so if you don't change your passport before you apply for FLR(M) you will have to use your old name on your application and your visa will be in your old name.
If you would like your visa to be in your married name you will need to renew your passport in your new name before you apply for FLR(M).
If you think you would want to change your name on your passport in the next 2.5 years, I would do it before applying for FLR(M) as it's expensive and a big hassle to do it after you get the visa - you will only have 3 months from changing the passport to change the name on your visa, or you can be fined £1,000 and potentially have your visa curtailed. It costs about £220 to do it and can take up to a few months, during which time you will be without your passport and visa.
Sent from my iPhone using Tapatalk