I would say retiring at state pension age with £100k and being frugal is easily done, but retiring much earlier would be a challenge. I got higher figures for a healthy 55yo on the Aviva calculator with that pot (around £3300) and it increased to £4500 for a 65yo. You can take the 25% of course which would give £12500 pa to live on for the two years upto 67, but then your annuity will be lower at around £3400. The 25% wouldn't be enough for more than 2 years I wouldn't have thought.
Regarding value, you just take your pension pot and divide it by the number of years you expect to live. If you're 55 with a £100k pot, with avg life expectany at 82, then £100k/27 = £3700 pa. You get less if you're healthy as you will probably live another 5 years, so £100k/32 = £3125.
Figures will obviously go down if you add dependants, protect your fund payments if the worst happens etc..