Below is the number on the back of my old SIM and it won't activate, would love to use it as just about to go on trip and don't have time to physically recieve a replacement. Any help much appreciated!
The 3Ggg1 indicates that it is a very old sim. If it has never previously been activated, it should still be possible to activate it, but it is more likely to take 24 hours than the usual 30 minutes because giffgaff does not keep records of those old sims online.
If it has previously been activated then it would not be possible to activate it a second time.
But personally I wouldn't mess around with that sim, especially if you have a 4G phone. Although 3G sims can be used on 4G in the UK, they connect initially at 3G and then switch up to 4G, therefore taking longer to connect. Overseas, I wouldn't be sure you'd get 4G at all even if it's available. So, better to pop out to a nearby supermarket or argos where you will get a fresh, unactivated giffgaff sim for about a pound. Note however that the sim will still require activation as normal -- they do not contain any minutes, data or text.
Note also that using a UK sim in EU/EEA without significant UK usage in the last two months may mean you don't get full goodybag-only service for some or part of your trip. In that case, you may need some credit outsideyour goodybag to pay the fair use supplement of 3.2p/minute, 1p/text, 0.47p/MB -- though it makes no difference if you're travelling to a non-EEA country, in which case goodybags don't apply anyway.