Hi OP
Please don't stress about this.
If your car seat is too tilted on the base, simply take it off the base and fit it with seatbelt. The instruction manual will show you how to do this or there might be a video online.
When fitted with a seatbelt, you can tilt the seat back much more to accommodate any tilt in your vehicle seats. If your back seats are very slanted, and the isofix points are low down then you'll have this problem with most isofix rear facing seats, for this stage and the next. It's worth being aware of this issue as it does affect a significant minority of cars.
You don't need to buy a new seat, as your baby's head and neck control will improve and so they won't have head slump any more as they get older and you may even be able to use the base again. You might even find that at 6 weeks this has already improved from when your baby was newborn. Plenty of newborns fit poorly in car seats, the smallest model that the car seat manufacturers work with is based on an average 6 week old baby. They grow a lot in the first few weeks so 6 weeks is often much much better than newborn.
In addition, even if your baby's head does slump, it is not dangerous when this is for a short amount of time. It is recommended for babies to spend no longer than 2 hours in a car seat for this reason, but up to 2 hours at 6 weeks is considered safe. If you want to be even more cautious, you could keep journeys as short as possible and transfer to a flat surface such as pram carrycot when you have stopped the car.
Some further tips to avoid head slump are to check that the shoulder straps are in the right position, not too low and not too high. The straps should be level with your child's shoulders or the bottom of the headrest level with the bottom of the chin. You should be able to get this checked via email with Silver Cross or at the store where you purchased the travel system. And ensure that you're placing the baby as low as you can in the seat. It's OK to spend some time sitting with the seat at home, get your forearms under each side of the baby and practice moving them up and down to see which position is right. The seat will be less upright when placed on the floor and so you can monitor your baby's position. Lower in the seat tends to help with head flop. It's OK if that means that their bum isn't right back in the "seat" - infant carriers are designed to be laid down in rather than sat in anyway. There is also an extra insert for the Simplicity Car Seat, which can be purchased, if yours didn't come with this, that may help.
It is important that the straps are tight and snug to your baby's body, for safety reasons. Tight straps also help prevent head slump by keeping baby correctly positioned in the seat. Babies are not uncomfortable in tight straps; they actually tend to find this comforting - think about how swaddling is soothing to newborns. If you can slip one finger under the straps this is OK - to be able to slip more than one finger between strap and baby means it is too loose.
Isofix bases tend to be brand specific and sometimes also model specific, so if you did get a different seat then you would need a different base if you want to use it with a base. If it's that your seats are slanted, then a new seat might not solve the problem anyway, you'd want to seek out a model that is specifically designed with sloping seats in mind.