If you have a 5 Pence coin that is the same diameter as a 1988 10 Pence coin, yes, it is rare.
The largest 5 Pence coin was 1.5 mm smaller than the smallest 10 Pence coin.
You may have a 5 Pence coin struck on a 10 Pence blank.
Any coin with a "genuine" minting flaw would have some value, above the usual, as a collectible coin.
Genuinely flawed coins are not necessarily known about or documented until somebody turns up with one, since they are an "accident" of the minting process, and have escaped detection during quality control at the mint therefore, a valuation cannot be anticipated.
A reputable coin dealer should be able to identify and confirm the coin as genuine and make a valuation.
It is in common circulation and worth 2 British Pence, or about 3 cents US/Canadian. It is a common coin and worth no more than face value.
2 pence GBP in 1988 had the same buying power as $0.06 USD.
1988 ;)
32 pence per litre in 1988
British Caledonian ended in 1988.
British Airtours ended in 1988.
A Very British Coup - 1988 is rated/received certificates of: Australia:PG
The New British Poetry was created in 1988.
These coins are still potentially in circulation so, unless they are part of a Proof or Uncirculated mint set or are individual Proof or Uncirculated coins and in absolute mint condition, they are worth 20 Pence in Gibraltar. The Gibraltar currency (GIP) is pegged at par with the British Pound Sterling (GBP) and the Gibraltar decimal coinage is identical in dimensions and composition to the equivalent British coins, being struck on the same blanks. Gibraltar currency is "legal tender" only in Gibraltar.
One Pound GBP in 1988 had the purchasing power of about £1.80 GBP today.
The larger pre-1990 5 Pence coins were withdrawn from circulation and demonetised in 1991. So, unless they are part of a Proof or uncirculated mint set or are individual Proof or uncirculated coins and in absolute mint condition, they have little or no value. A British 1987 cupro-nickel 5 Pence coin (QE II)(Proof FDC), uncirculated and in absolute mint condition could fetch up to £2 GBP. A reputable coin dealer will be able to give a more accurate valuation.
The British Half Penny was no longer issued after 1984.