A landlord can decide whether or not they wish to allow pets in their properties. There is no obligation for a landlord to accept pets. However, if they do they can raise the rent but they cannot charge you a deposit higher than 5 weeks' rent as per the Tenant Fees Act 2019.
Landlords can no longer issue blanket bans on pets. Landlords will need to object in writing within 28 days of a written pet request from a tenant and provide a good reason. You should make a request to your landlord in writing and avoid sneaking your pet into the property as you will be in breach of your tenancy agreement where you have not taken reasonable steps to gain consent prior to introducing a pet into the property.