Mini-MARVEL - Mitochondrial Antioxidant Therapy in Ulcerative Colitis

Inclusion Criteria: * Age 6 to17 years in a PIBD service * Able and willing to give informed consent if aged 16-17 years of age; parents or guardian able and willing to give informed consent if a child of age 15 years and under * Confirmed UC as documented in the medical notes * Mild-moderate severity (PUCAI score 10-55) * (i) If Incident (newly diagnosed) case (biopsy confirmation of suspected UC at endoscopy) or within 2 weeks of diagnosis - no anti-inflammatory medication or if started on 5-ASA on day of endoscopy for presumed UC (ii) If Prevalent case - No medication or stable dose of existing medicine (oral/topical 5-ASA for \>2 weeks at screening; thiopurine for \>4 weeks at screening) Exclusion Criteria: * Inability to swallow capsules * Young person (16-17 years of age) with inability to provide consent due to a lack of capacity * Crohn's disease (CD) or IBD unclassified (IBDU) as documented in the medical notes * Evidence of acute severe UC (ASUC) - ASUC is a medical emergency in children and young people with UC who are clinically unwell with marked diarrhoea, haematochezia and abdominal pain, and by definition have a paediatric UC activity index (PUCAI) score of at least 65 points * Physician judgement that the subject is likely to require hospitalisation for medical care or surgical intervention of any kind for UC (e.g. colectomy) within 12 weeks of baseline * Evidence of current (or previous in last 12 months) toxic megacolon (using defined paediatric criteria - radiographic evidence of transverse colon diameter ≥56 mm (or \>40mm in those \<10 years) PLUS evidence of systemic toxicity (such as: fever \>38 degrees Celsius, tachycardia (heart rate \>2 SD above mean for age), dehydration, electrolyte disturbance (sodium, potassium or chloride), altered level of consciousness, coma, hypotension or shock) or bowel perforation * Infective colitis in UC (C\&S, C. difficile toxin positive or virology at screening) * Proctitis (inflammation confined to the rectum, with no proximal extension to the sigmoid) for incident cases after colonoscopy or prevalent cases at most recent colonoscopy * UC with Primary Sclerosing Cholangitis (PSC) - PSC is a chronic disease characterized by inflammation and scarring of the bile ducts resulting in strictures of the biliary tree; most cases in children are associated with IBD. The diagnosis is made by liver imaging with MRI/MRCP. * Age \<18 years but in adult IBD service * Intravenous corticosteroids for treatment of colitis within 8 weeks of screening * Current or previous exposure to tacrolimus, cyclosporin, or mycophenolate, * Current or previous exposure to biological therapy (anti-TNF, vedolizumab or ustekinumab) and oral JAK-inhibitors (tofacitinib) * Subjects with current barriers in language or communication that in the judgement of local PI will impede the completion of the trial. * Female patient with child-bearing potential who does not wish to use a medically acceptable method of contraception throughout the treatment period and for 1 month after discontinuation of treatment. (Appropriate methods of contraception include IUD, oral contraceptive, subdermal implant and double barrier (condom with a contraceptive sponge or contraceptive pessary) * Pregnancy (current declared or positive urine pregnancy test) or attempting to become pregnant during trial period) or breastfeeding * A history of overdose, or significant active mental health problems. * Monogenic IBD (an autosomal condition with an IBD-like phenotype; most common in those with IBD diagnosed \<2 years of age) * Subjects with current - or recent history of - severe, progressive, or uncontrolled renal, hepatic, haematological, gastrointestinal, metabolic (including uncontrolled hypercholesterolemia), endocrine, pulmonary, cardiac, neurological disease * Subjects with a known allergy/contraindication to MitoQ. * Subjects currently taking any products containing Mitoquinol mesylate (Coenzyme Q10) or any products containing Coenzyme derivatives such as Coenzyme A (CoA, SCoA, CoASH). If subjects are on these products, they can enter the trial after a 7-day washout period.