The scorer node for numerical targets is already there and provided us with the RMSE statistic to evaluate the model.
You could try to read more about the prediction of numerical targets in this collection of entries:
Also if your assignment is from a university or other educational institution you might want to read the provided literature or ask your professor or tutor which statistic is the preferred/expected one. RMSE is widely used and also often the decisive statistic in Kaggle competitions.
Another note: the models compared in this example are used in their most basic configuration. All the good stuff like normalisation, feature engineering and hyper parameter tuning are not used (yet). But the question is if with so few data points you would get better results.
Also you could legitimately try to predict each area with a time series of its own. But since all parts of the UK are economically connected the model might benefit from having them together in one dataset. And we left the area information there for the model to be used. Another approach could be to leave the area out.
But all this is an addition. You should first concentrate on getting your setting and ‘business question’ right.