Mould is a serious issues as it can lead to respiratory issues. Sewage problems would be an environmental health issue (you do need to reach your environmental health department at the council to report this).
In UK law, landlords must ensure the property is in good repair under Section 11 to 16 of the Landlord and Tenant Act 1985. Further, a recent law came in to force to provide more protection to tenants, which is the Homes (Fitness for Human Habitation) Act 2018 is now in force - the house or flat needs to be healthy, safe and fit for human habitation (free from things that could cause serious harm). So the issues you mention which afflict the property would put the landlord in breach of this law.
If they do not, they face prosecution by the local council and can be fined. It would also be a breach of contract on their part. The only situations where a tenancy can be ended are, you finish the tenancy term or there is a break clause meaning you can end the tenancy part way through. Or the landlord agrees to you leaving early - usually if another tenant can be found. You could terminate but you would need evidence of repeated requests to the landlord to fix and they do nothing. If you terminated then they could sue and you may have to defend the claim in a civil court.
The Local Authority Environmental Health Officers have powers under the Housing Act 2004. They have a duty to ensure that properties in their area are in a habitable condition, and will serve improvement notices on landlords of properties which are assessed, under the new Housing Health and Safety Rating System as having ‘category 1 hazards’.
Similar powers are available to Local Authorities under the Environmental Protection Act 1990 if the property is considered a threat to public health.
I would therefore recommend that you contact your local council (the housing department) in the first instance to report the matter and ask them if they can send someone out to do an emergency repair - if so, they will do the work and they will bill the landlord themselves.
Yours can be reached by telephone on 01753 475111
If for some reason the council cannot help, you would either have the option of paying it yourself and withholding rent or to instruct a tradesperson to come out and fix the problem and you then pursue the landlord for the money. You can also claim any loss of earnings as you said you had to take time off work.
If the landlord does not pay or if they refuse for whatever reason, you can sue them in the county court for your losses on the money claim site (www.moneyclaim.gov.uk) although I recommend sending the landlord a letter before action to warn them of your intentions first and allow them 14 days to reimburse you. If they do not reimburse, you are free to issue a claim.