​The show has  5 classes for roses
The RHS advice on judging is comprehensive, this is just a summary to get you started

Stem: An original new growth with no lateral stems
Double bloom: Has more than 20 petals
Semi Double: Between 8 & 20 petals
Single Bloom: has less than 8 petals
Miniature Rose: Tiny in every respect from flowers to leaves
Cluster flowered - Two or more blooms, gracefully arranged & spaced
Side buds allowed except in classes for specimen blooms or 3 stage roses
Additional Foliage: Not allowed 
Wiring: A single wire may be used to hold a large flowered rose head erect

Defects:
Irregular outline, fewer than average number of petals, petals, split, stained or damaged
immature or over developed blooms. Stems weak and twisted, poor thin foliage, untidy presentation
Flowers either crushed together or too widely spaced.

Judging roses

Marking Advice
Form & Size of individual blooms,
form of cluster, stage of development                     5 points
Freshness, brilliance & purity of colour                   5 points
Stems & Foliage                                                      3 points
Presentation                                                            7 points
​TOTAL                                                                   20 points