The following preps sent a considerable number of boys to Harrow in the past years:
| London preps | 2017 |
| Northcote Lodge, Clapham | 7 |
| The Hall School | 4 |
| Eaton House the Manor | 3 |
| Thomas’s Battersea | 3 |
| Dulwich Prep | 3 |
| Wetherby Prep | 3 |
| Arnold House | 2 |
| Newton Prep | 1 |
| Hill House | 1 |
| WCCS | 1 |
| St Anthony’s Prep | 1 |
| Boarding preps | |
| Caldicott | 10 |
| Summerfields Oxford | 8 |
| Papplewick | 4 |
| Cheam School | 3 |
| Elstree School | 3 |
| Ludgrove | 2 |
| Horris Hill | 2 |
| Sunningdale | 2 |