It looks like nice W-R gold name model. 28-30inch is pretty much standard length in UK for guns used on driven birds. Mid bead and tight chokes...Maybe an inquiry to WR will shed some light.

Normal game shooting shells are usually 28-32 grams for 12 bore, start out in early season with 28 gram 7's and then move to 30 or 32 gram 5's ( about US6) end of season some guns go for 30-32 gram 4's (US5).

20 are usually loaded with 23-25 gram, 28's are considered heavy among some.