The "Pension" records are simply those records that were with the Ministry of Pensions at the time when the Blitz saw the destruction of the bulk of the WW1 service records. After the war the War Office asked all departments who had relevant files on WW1 soldiers to send them back in an attempt to piece together at least some of what had been lost in the Blitz. As far as I am aware the only department that had any records was the Ministry of Pensions, who duly obliged and sent the records back.
As a result there were two sets of files, the so called "burnt records" - what was left after the fire and water damage - and the "Pension" records - the ones from the Ministry of Pensions. Both sets of records were made available to Ancestry to create the digitalised sets that are now on their website. If you go to Kew all that will happen is that you will be directed to a computer which is able to access Ancestry for free. I suppose if you really wanted to you could ask to see the original microfilmed records, but there wouldn't be a lot of point as all you would get was what was sent to Ancestry in the first place.
As far as entitlement to a pension is concerned the quick answer is that not everyone was eligible for a WW1 pension. If a man was wounded, gassed or suffered illness which was attributable to his war service then he was sent before a Medical Board. If, in their opinion, there was sufficient cause, then a pension would be set, usually a few shillings paid weekly. All of the ones that I have seen were time limited - that is they were awarded for a set period of time, usually 26 or 52 weeks, after which it was back in front of the Medical Board to see if the pension was still needed.
Hope that helps a bit,
jds1949
