Unfortunately it is inevitable that, with so many runners, some people will get injured in training for a race. What happens next depends on how you got your place and what race you are doing.
If you are using your 'own place' to run for Oxfam (i.e. you obtained it directly from the organisers of that race) you simply need to let them know you can no longer take part. From there it is their decision as to whether you can take up your place the following year. We'd also appreciate it if you could let us know so we know how many runners to expect on the day.
If you got a 'charity place' in the race (i.e. you obtained your place through Oxfam) then you can usually defer that place to the following year as long as you let us know before the registration deadline for that race. If you let us know after the registration deadline then unfortunately we cannot guarantee a deferral.
We also cannot guarantee a deferral for those dropping out of the London Marathon, whatever stage they're at in the lead up to the race. This is because we have an ever-decreasing number of places available.