Exact diameters of some common UK coins are:
LSD: Farthing 20mm, Halfpenny 26mm, Penny 30.81mm, Silver threepence 16mm, Brass threepence 21mm, Sixpence 19mm, Shillings 23.1mm, Florin 28.5mm, Half Crown 32mm, Crown 38.6mm.
Decimal: Halfpenny 17mm, Penny 20.32mm, Twopence 25.91mm, Old Five Pence 23.5mm (pre 1990), New Five Pence 18mm, Old Ten Pence 28.5mm (pre 1992), New Ten Pence 24.5mm, Twenty Pence 21.4mm, Old Fifty Pence (pre 1997) 30mm, New Fifty Pence 27.3mm, One Pound 22.5mm, Two Pound 28.4mm, Crown 38.6mm.
23mm are good for the 12 sided £1 coin
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With capsules you need to go up to the nearest mm. Hence two pound 28.4mm will need a 29mm capsule. Unfortunately, there will be a bit of rattle. If you are using a quadrum, then you can use a 28mm size as the foam can expands a bit with no problem. Hence no rattle and eliminates the risk of friction between the coin and plastic. I personally use quadrums even though you can't see the edges of the coins.