Is there any particular custom as to which order platforms are numbered on stations?

I have always assumed that Platform 1 would be the platform closest to the main entrance to the station, and in the vast majority of cases that i know of, this does seem to be the case. However, there are exceptions - two that I know of straight away, because they are stations I have used quite a lot, are Bath Spa and Hereford. At both of these, platform 1 seems to be located farthest away from the main entrance.

I was almost caught out by this at Bath Spa last week when the line between Bath and Bristol was closed. I went to catch the train to Westbury, glanced up at the screen in the station lobby to check the train was on time, and noted it was on platform 1. As it always goes from the platform closest to the main entrance, this just lodged in my brain as meaning it was running from its normal platform. It was only after I got up there with my bike, and found it wasn't coming in on that platform, that I realised that it was going from what is usually the Bristol-bound platform, and that is platform 1. A very hurried rush to get back down to the subway and back up to the other platform ensued.

 
1 is at the far side at Swindon too ...

 
The most consistent thing I can think of is that at termini the platforms often start with 1 on the left when facing platforms from the concourse.

On the GWML it's common but not completely consistent for platforms to start with 1 on the down side – maybe just something that's evolved, or maybe an attempt at consistency and aligned with the order at PAD. I suppose there's some merit in consistency, as anyone who knows the rough platform layouts from PAD to RDG, for example, can work out the platform numbering without learning them all.  That's probably only a small advantage, but in the 'nice to know' category for some.

Of course, in some cases the numbering is intentionally swapped. When additional platforms were built at BPW, swapping the order meant all existing platforms had either the same number or a completely new number when changes occurred, so there was less of an issue with passengers going to the wrong platform. The order was also swapped at CDQ when two new platforms found their way in, but I think that was just to keep the numbering neat, more than to avoid confusion.

 
Platform 1 at Newton Abbot is furthest from the main entrance and many customers are unaware of the layout.

 
At Nailsea & Backwell, my local station, the 'main entrance', up the steps towards the ticket office and the one used by many commuters into Bristol, is platform 2.

Platform 1 is opposite, for those travelling west towards Weston, Taunton, etc.  There is no ticket office on, nor disabled access to, platform 1.

CfN. 

 
At my local station there is no ambiguity whatsoever. But ( and I’m sure I’m not alone in this!) I could live with the ambiguity of having more than one platform if it meant a more frequent and reliable service…

 
As Hafren rightly says, Platform on the GWML are generally numbered with Platform 1 being on the down direction side.

There are of course a number of exceptions to this rule such as at Swindon, Oxford, Theale, Newbury Racecourse, Hungerford, Castle Cary, Lostwithiel and Penzance.

 
... to name just a few such exceptions to the rule. 

 
When did "simple" two platform stations, Evesham, for example, get platform numbers?  I know Evesham and Moreton in Marsh didn't have platform numbers in BR days, and I'm reasonably confident that places like Bridgwater, Tiverton Junction and Dawlish didn't, either.

 
When did simple single platform stations like Lymington Town get numbers? Presumably to feed the computer.

 

A couple of years ago I was on a train that failed at the type of local station that just has signs for 'Trains to X' in lieu of platform numbers, but which has numbers on the systems. The failed train was sitting there so I contacted the TOC via Whatsapp for an update. They told me the next train would make an extra call. It wasn't clear if the failed train would be moved or if the 'replacement' would run wrong line and stop at the other platform. So I asked the person on Whatsapp and they told  me the platform number, which I don't think was displayed anywhere on the station. Not exactly helpful to someone standing there! Only way to find out was to check online for the booked platform for other trains to deduce which was which, but I suspect in any case had it run wrong line the system would still have shown the platform it was booked to pass, so the info may not have helped unless the person personally knew the details.

An interesting case is Llanelli where I think platforms are numbered, but RTT shows the platforms by their line identities, UPL and DPL!