Most off-the-rack clothing fit issues (sleeve length, hem length, waist nipping) are 30-minute alterations costing £8-25. UK alterations services are cheap and underused. A £40 dress tailored properly looks better than a £200 dress that doesn't quite fit.
Alterations worth doing
Sleeve shortening on jackets and blouses. Trouser hem length. Skirt and dress hems. Waist nipping on trousers and dresses (single adjustment, £15-25). Shoulder reshaping on jackets (more skilled work, £40-80).
Where alterations don't help
Pieces that fundamentally don't suit your shape (return instead). Heavy structural changes to luxury pieces (often more expensive than the piece itself). Knitwear (hard to alter without ruining the knit).
Most local dry cleaners offer basic alterations. For complex work, find a specialist via Vinted or Reddit local recommendations. The cost-per-wear maths transforms when £40 pieces become custom-fit.