As the following is in place through all UK telephone suppliers re 5 digit sms numbers
United Kingdom Codes are five digits in length, mostly starting with 6 or 8. Codes starting 7 are used by charities.[11] The range of codes may be expanded in time to use other leading digits such as 4.
And as your sms competition number is based in the Republic of Ireland, then it's a different leading number i.e
Republic of Ireland
Short codes are five digits in length, and start with 5. The second digit generally indicates the maximum price, with 0 = completely free, 1 = standard text rate only, 3 = €0.60, and 7 having no maximum. Codes beginning 59 are ostensibly intended for adult services, but few if any of these codes are used.[4]
At present, if you're sending your sms to another country, IMO & experience, the telecom provider in the originating country doesn't recognise the "leading" number of the sms 5-digit code as a valid number in that country, and as such will not transmit the sms to the receiving number - thus giving a number unobtainable.