RSPB do not take in injured birds, they are a conservation charity rather than a wildlife santuary. Do you mean the RSPCA? They are the agency to contact if you find an injured bird, or else a local vet or wildlife sanctuary. See here. Often the RSPCA though do not have resources for injured gulls, there just isn't the funds, but they will be able to advise.
Was it definitely injured rather than just unable to fly? Sometimes baby birds fall out of nests and are not able to fly, in that case the best thing to do is leave them as the parents will usually be able to hear them. See here.
Hopefully it will be OK in the box overnight and the RSPCA will be able to give advice in the morning.
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