It won’t if your finances are kept separate. As soon as you make joint applications for credit or set up a joint bank account, however, then they will be linked.
Say you are applying for a mortgage in joint names; lenders will check the credit histories of both applicants. If your credit history is exemplary, then that may well mitigate the effect of your spouse’s bad credit but, as you are looked upon as jointly or severally responsible for the loan the bad credit will likely take precedence over their decision.
As always, make sure the details on record are accurate. Start by getting both applicants sets of credit records from all three of the UK’s credit referencing agencies, Experian, Callcredit and Equifax.