DIY solar is legal in the UK. It also disqualifies you from SEG, voids most warranties, and gives mortgage lenders a reason to discount the install. The savings rarely cover the losses.
Self-install saves £2,800–£4,500 in labour on a 4 kWp system but costs you SEG eligibility (no MCS = no export payments), most panel and inverter warranties (most require MCS commissioning), and 1–2 EPC bands of resale uplift. For all but a small minority of off-grid or hobbyist installs, MCS pays back the labour cost inside 4–6 years.