The reason a devalued Pound would be a bad thing is that it would mean increased exploitation of workers. Firstly, a devalued currency means that all of the things you need to import for workers subsistence rise in price, and wages tend only to rise to cover that increased cost gradually, and after a considerable time delay, during which real living standards thereby fall.

Secondly, what devaluation means is this. If currently a British car exchanges for a German car, if the value of the Pound falls, one German car exchanges for two British cars. That is why the British car becomes cheaper, and easier to sell. However, in order to import the one German car, or any other set of German, French, Italian etc. commodities we might want, it then becomes necessary for British workers to produce and sell two cars, where previously they only had to produce one. In other words, the devaluation means that British workers will have to work much harder, much longer, and enjoy lower living standards.

Its why nationalists like Farage and the Brexiters always favour such protectionist measures, rather than addressing the problem of lack of competitiveness due to lack of adequate capital investment.