Before jumping down the ACL path, a couple of questions need to be resolved to determine if TomTom (their service provider's which I assume is Vodaphone as they coincidentally are turning off their 2G network on 30 April 2018) closure of the 2G network is reasonable and that you have been left in the lurch...
The first is when did you purchase the GO5100/6100 TomTom....as the model runs from the 2G network amd it has been reported in the mass media that the 2G networks had a finite life?
Was the GO5100/6100 TomTom purchased before or after the 30 September 2016?
If the GO5100/6100 TomTom was purchased before the 30 September 2016, no-one (inc. TomTom) would have envisaged the closure of the 2G network on the 30 April 2018. In such case, TomTom may not be responsible under the ACL.
However, if the GO5100/6100 TomTom was purchased after the 30 September 2016, then on purchase of the GO5100/6100 TomTom, you should have been advised that the 2G traffic and speeed camera functions will not longer work after either 30 September 2017 or 30 April 2018 (depending on when you purchased and the information known at this time. If you were not advised on purchase, they you would have good grounds to request resolution under the ACL as the TomTom was not fit for purpose and you expected, based on the information at the point of sale, that the TomTom would have a life of greater than maximum of 19 months.
If you were advised at the point of sale of the imminent shutdown of the 2G network (or possibly new of this from reading mass media), then arguing resolution under the ACL may be problematic, as you were aware of the limited duration that the 2G functionality would exist and accepted this on purchase of the GO5100/6100 TomTom.
(Note: Vodaphone originally announced the shutdown of their 2G network on 30 September 2016 - the original shutdown date was 30 September 2017, but this was extended out to 30 April 2018).
Do all other (non-2G) functions of the GO5100/6100 TomTom operate as required/in accordance with the manufacturer's specifications?
If they do, then it will be difficult to argue, especially if you were aware of the 2G shutdown on purchase, that the GO5100/6100 TomTom was not fit for purpose or had a quality issue.
If the TomTom is several years old (say bought 3+ years ago), it would be seen reasonable that the device's life has ceased for the 2G functionality. I expect that based on changes to technology and also ability for smartphones to also present similar traffic and speed camera information through apps, that this part of the TomTom may be nearing the point of redundancy. While I hate to see any technology redundancies, unfortunately it is a fact of life in the age we live.
Possibly the 50% off offer is reasonable for those who rely on TomToms to perform live traffic and speed camera functions. I expect however, if you have a smartphone, free/paid apps will be a lot cheaper than what TomTom is offering.
Choice has done recent reviews of mapping apps and there are also several forum thread which also discuss these apps.