It was definitely not a pub in 1823 and, as you suggest, the odds are very much on it being opened under the terms of the Beer Act - so after 1830. I would also guess that it was after 1841 - the census that year suggests that the occupant was a tailor. However, it is impossible to be 100% sure about the correlation between house and census entry! It is also very possible that the tailor was selling beer as a sideline, many/most beerhouse keepers had a day job as well as the beer sales.
Having looked into my records I am a little more confused - it is also possible that 54 became a beerhouse in the 1850s:
In 1851 William Whitehead 'sergeant grenadier guards' is living in the house that would appear to be 53 Northbrook St (ie next door to the George & Dragon) and the same tailor is still at 54. The 1851 for Newbury is not very reliable in terms of locating houses so I would not deem this a reliable source in terms of address - but it does tie in with the 1841 quite nicely.
In Slater's Directory of 1852 William Whitehead is listed as a beer retailer at 53 Northbrook St. Directories are hardly the most reliable source, 53 could easily be a typesetting error.
In 1861 Whitehead is a beerhouse keeper of The Grenadier at 54 Northbrook St. This is clear and unambiguous. The 1861 census for this side of Northbrook St is really excellent - and gives good evidence of the consistency of numbers - for instance the Monument Inn is No 57 - as it is today.
So the questions are whether Whitehead was already selling beer in 1851 and was he at 53 or 54. It is not impossible that he moved next door - there are other instances in Newbury of beer houses moving like this.
It would be nice if he appeared in other 1850s directories - but he doesn't. However the Lamberts N and Mary do - running a beerhouse called the Independent Soldier at 53 Northbrook St.
It would not be the first time that a pub moved next door for some reason (better premises, falling out with the owners) and someone else moved in and used a similar name. But it does add to the confusion!
At the moment I am tending towards the timeline being:
1. Whitehead leaves the army in the 1840s and settles in Newbury at 53 Northbrook St - he obtains a beerhouse license and starts trading.
2. He leaves 53 in 1852-3 and is replaced by the Lamberts who run the pub as The Independent Soldiers, This may well have been the name while Whitehead was there - I simply haven't found it as anything other than an anonymous beerhouse in that period.
3. Whitehead returns to Newbury before 1861, perhaps he never left? Opening the Grenadier at 54. Somehow I doubt he would be missing from at least 3 directories if he was in town in the mid 1850s.
4. The Lamberts leave and the Independent Soldier closes in the 1855-1861 period.
5. Whitehead moves on in 1869-1871 period, replaced by Joseph Fidgett, who renames the pub The Drummers - though it also appears as the Grenadier and as the Sergeant.
6 Fidgett replaced by William Lye in 1881, name is definitely The Sergeant.
7 Lye replaced by William Marshall in 1883
7 Marshall replaced by Henry Crook in Nov 1883 - who is soon convicted of keeping a disorderly house (prostitution was rife - the neighbours were talking of frosting the glass at the back of their house so they wouldn't have to see what was going on behind The Sergeant). At the Annual Licensing Sessions (Oct 1884) the local magistrates refuse to renew the license despite the brewery's (Nutley's) promise to get rid of Crook. Building reverts to use as shop.
Not one of Newbury longest living pubs!
The issue of there being two pubs at 53 and then 54 Northbrook St is probably the most contentious issue - and I may well be wrong, the premises may have been the same, the numbering may have changed. In the 1871 census the numbers are definitely awry at that end of the street - I need to look a lot more closely before I would be convinced that the census enumerator did not mess it up!