This is really common, unfortunately!
The simplest, safest, but most expensive solution is to get a car seat with impact shield instead of 5-point harness. E.g. The Kiddy Comfort Pro or the Mamas & Papas Cybex Pallas. (These two both convert to high backed boosters as well so will last you longer - the Kiddy Infinity Pro which is just for her age group until about age 4, is around £60 cheaper). The arms are free but they are still very safe and there is no way they can reach to undo the straps.
The next best option is to teach her not to do it, how successful this is depends on how stubborn she is but also what her understanding is. At 21 months it's hard to say without knowing the child.
Or you can make it less easy for her to get out, there are a few simple things you can do, like making sure the straps are tight (which you have done) and making sure she is not wearing bulky clothes in the car (especially not coats) but ultimately, 5-point harnesses are quite badly designed because it's always going to be possible for the child to slip their arms out through them.
You can add something extra to the car seat - you can buy a clip which keeps the straps together across the chest for example, or use a normal set of reins and put these on her backwards and thread the car seat straps through them. The problem with this is that UK carseats are specifically designed to only have one button to press to release the harness, which means in the case of an accident if you couldn't get to the seat a stranger can easily open it up and take your DD to safety. Obviously this is a really unlikely situation, but something that is worth considering.