So worth bearing in mind, invariably you will do maths in the style of A-level Maths in any degree (broadly all falling under the umbrella of "mathematical methods"). A joint honours in something and maths at degree level is thus a combination of "maths-degree maths" and the other subject (which will include those mathematical methods anyway).
The kind of maths encountered in degree level is very different to the kind of maths in A-level, so it's worth recognising that and making sure that is what you want to be doing. It's very abstract (even the applied maths portions of a maths degree are usually necessarily abstract), and the pure maths content is essentially entirely proof based (barring some basic computational exercises in linear algebra or something early on I guess).
If you just are worried you are "dropping" maths, you don't need to worry as effectively any science degree (certainly any physical science degree, e.g. chemistry) will be necessarily mathematical. That said there are some areas of maths that are very much "maths-degree maths" which have bearing on chemistry - I gather group theory has a number of uses in inorganic chemistry for example, and if you wanted to go down the theoretical physical chemistry route then much of the same maths physicists use (including those more pure areas potentially) may be applicable (so more formally abstract introductions to linear algebra and complex numbers/analysis might be quite productive). So could be a really useful background for specific PhD projects in those kind of realms - not sure it would be as much of a big deal for organic or synthetic chemistry though?
However that is all very much academic. In terms of non-academic careers, it's pretty much a wash - most generalist graduate schemes don't care what you studied at degree level. For more specialist roles some specific background may be needed, so for going into data science or aiming to become a quant a background in maths at degree level (in the former case leaning hard onto the stats side, in the latter case something more computational although quite possibly stats-probability related) might be useful (although bear in mind to become a quant you'd usually need to be doing a PhD in a heavily numerate or computational subject e.g. maths/physics/CS/maybe engineering). There are some other roles where a joint honours in maths and chemistry might open a couple doors otherwise not available (e.g. actuarial work perhaps, again if leaning on the stats-probability side I'd imagine).