There is some alarming misinformation about these types of seats on social media. Honestly they aren't as bad as some make them out to be, and I think the Cosmo is a bit more substantial than some of the other Nania/other budget brand seats. It has deeper side wings and the plastic seems a little bit thicker. The isofix version was tested by the company that does the Which? crash tests in 2009 and it got an average safety score (the seatbelt version is likely to be a little bit worse). Bear in mind that the standards for this test have increased over time, so an average in 2009 is a bit lower than an average today.
The main problem with this seat is that, at 18 months, it is a lot safer for your baby to be rear facing, but it's likely that she is already over the 10kg rear facing limit for this seat. The other problem with these seats is that they are (simply put) very cheaply made, with price being the highest consideration. The safety features are very basic, about equivalent to car seats that were around in the late 1990s. Car seats from brands which have more of a safety focus have more features such as allowing longer rear facing (up to 13kg, 18kg or longer), seatbelt tensioners which tighten the (car) seatbelt for you, or isofix fitting, more sophisticated protection from side impacts, better, more fitted inserts to ensure a more snug fit (reduces movement in a crash), a central adjust safety harness rather than the individual tighteners, energy-absorbing materials to help reduce crash forces acting on the child, and many have some kind of system to reduce forward rotation, which is especially dangerous with forward facing seats.
If she does happen to be under 10kg, you could turn her rear facing today and keep her there until she is that weight/while you decide what to do. If she's over that, which is likely, then the best thing to do would be to buy another seat that can keep her rear facing up to 18kg (or longer if you desire, though the longer lasting seats are more expensive).
However, if it is not an option to get another seat, and I totally understand that it might not be, the best way to keep her as safe as possible in this seat is to ensure that you're using it correctly every time, which means checking that the safety harness is adjusted properly (should be just level with her shoulders) and always very snug to her body, without bulky clothing getting in the way. Check the fitting of the seat is correct by looking in the manual and also at the fitting video from the manufacturer. At every step after you plug in the seatbelt, kneel your weight into the seat, which allows you to get it much much tighter, and pull the seatbelt tight through the seat routings and through the place where it clicks into the buckle before you fasten the lockoff clips. The kneeling/pulling tight is not always shown in the instructions, but it's always something you can do with a belt fitted seat, unless it has tensioners, then you probably don't need to.
Once she has outgrown the safety harness on the seat I would get rid of it and buy a new high backed booster, I would not recommend using this seat in the high backed booster (group 2) mode because the belt fit is not very good. However, it's OK (though basic) as a harnessed seat.
If you did want a recommendation of a different seat I'd look at Graco Extend LX R129. This is very affordable at £100 but it has several improvements, it can rear face up to 18kg/105cm which is about 4 years old. It also allows forward facing after 15 months if you prefer this. It meets the newer R129 safety rating which has stricter requirements for side impact protection and forward rotation/protection. It has a large headrest for comfort and safety.
Or for around £120-140 you can get a purely forward facing seat which has either a belt tensioner for tighter seatbelt fit or is isofix fitted. For example Maxi Cosi Tobi, Maxi Cosi Priori, Cozy n Safe Excalibur, Safety 1st Everfix.
Or there are a few isofix, rotating (rear and forward facing) seats around the £150/160 mark by Joie and Graco which are also good options and would be a substantial improvement even if used forward facing.