Charles,
2:45 gets you entry into the AAA Marathon Championships held in conjunction with London Marathon. Basically guaranteed entry, nice loos and get to start at the front. Suitable target for you as well.
Alternately sub 75 half does the job as well. Following on from that sub 70 half gets you preferrential/elite entry at many races.
At your level you could also look at qualifying to run for your county in intercounties XC etc - this would be diffcult but likely achievable (depending on your county).
Final point is - don't set too many limits, a few minutes might be your limit, equally it might be in the low 2:20s - you won't know until you try.
Best!
Hobbling