Running a race using somebody else's number without informing the organiser is strictly prohibited.
The race organiser may not allow number transfers.
Running in somebody else's name apart from being against the rules damages the integrity of the results, can affect team results and age category winners and have health and safety implications in the event of an accident. Any member known to run in another runner's number may be subject to race sanctions. You may also be banned from taking part in other races as outlined below.
RULE 143 S 2 UKA SUPPLEMENT - ATHLETE BIBS (PREVIOUSLY UKA RULE 18, 103 & 209) (1) Every athlete shall be provided with at least one bib which shall normally be worn on the breast and show a suitable identification number or name. (2) Road Races (i) There should be provision for medical information and contact details on the reverse of the bib for prior completion by the athlete. (ii) In Road Relays every athlete must wear the supplied bib(s), which must serve to identify the team and should also identify the stage that the athlete is running. (iii) Numbers are issued to the individual athlete completing the application form except with the specific authority of the Competition Provider. Athletes who receive transferred numbers without permission will be disqualified from the race. Both runners will be subject to disciplinary action by the appropriate National Association. (vi) A runner must retire immediately if ordered to do so by a medical officer of the race. (v) Runners who retire from a race must not remove their bib until they have reported to an official as having retired from the race.
If you are in breach of the above two laws or indeed any other in the rulebook, the race organiser can report you to England Athletics who can decide any further punishment.