Aside: There have been a few questions regarding the legality of using English coins for this purpose, i.e. is it an offence to effectively deface legal tender bearing the image of the Monarch? I did my best searching around the internet and found this section apparently from legislation dating to 1973:
17. Whosoever shall deface any current gold, silver, or copper coin, by stamping thereon any names or words, whether such coin shall or shall not be thereby diminished or lightened, shall be guilty of a misdemeanour, and being convicted thereof, shall be liable, at the discretion of the court, to be imprisoned for any term not exceeding one year, with or without hard labour.
18. No tender or payment in money made in any gold, silver or copper coin so defaced by stamping, as is mentioned in section 17, or defaced by piercing, or in any other manner shall be allowed to be a legal tender; and whosoever shall tender, utter, or put off any coin so defaced shall, on summary conviction thereof, be liable to a fine not exceeding four dollars:
Provided, that it shall not be lawful for any person to proceed for any such last-mentioned penalty without the consent of Her Majesty’s Director of Public Prosecutions.
Obviously some of the wording is a little funny and I’m not sure if any of this still stands today, but it would appear that other people have tried to ask the same question before and received no response from the Royal Mint. Chances are that it is still illegal, but barring doing anything on a grand scale involving thousands or millions of coins in circulation (such as advertising on them) the justice system probably have better things to be worrying about. End aside.
So, it turns out that four English pennies (superglued into place to stop them from falling down or out of position and potentially screwing things up) is the ideal amount. This obviously makes pulling the slide back to cock the gun a little harder, but is worth it for the satisfying crack that the gun now makes every time it fires with a fairly noticeable amount more power. Very happy with this mod.