Hi Derek,
Unfortunately your Grandfather's Attestation and service records don't appear on the online documents viewable with Ancestry.com.
Looking at the medals he was awarded, the War Medal and the Victory Medal, he served as a Private with the London Regiment, with overseas service as follows:
15th. (City of London) Battalion (P.W.O Civil Service Rifles
7.2.1916 to 19. 9.1916 Regiment Service No. 4100
7. 4.1918 to 19. 4.1918 3rd. Battalion London Rifles. Service No. 279039
20. 4.1918 to 13. 5.1918 East Surrey Regiment. Service No. 678271
14. 5.1918 to 23. 8.1918 15th. Battalion London Rifles. Service No. 531628 (note the number difference to your record).
He was also awarded the Silver War Badge which records that he Enlisted 19. 6.1915 and was Discharged for Wounds age 28 on
24. 2.1919 .
He was probably temporarily attached to the E. Surrey Rifles to make up numbers for a particular operation, as were some others from his Battalion (which also happened to one of my relatives)
In the Absent Voters Records for 1918, he is listed as a Lance Corporal. I presume that his wife was living with her Mother at the time, as George's Father in Law is also shown as being an Absent Voter from the same address (with the West Riding Regiment).
I hope that this gives you some information, but you will probably have to try the National Archives for further help, although I would think that not every record has been copied, or filed under the correct heading. Good luck !