Hi I'm a Nursery Advisor with over five years of experience and would by far reccommend the Maxi Cosi Tobi for a Group 1 carseat.
Its a Group 1 carseat that, I think, if you don't have Isofix is the best. The reason being is that the seat is held in two positions around the front of the carseat rather than one position, at the back, like other seats within its grouping. Being held in such a way gives the carseat a really sturdy fit. It also has a built in permanent neck support which when moved up adjusts the position of the shoulder straps so you'll never ever have to re-thread them through.
As for your comment about the straps unfortnuatly with every carseat, not just the Tobi, after a while your child will figure out how to undo the straps as they are just getting older, smarter and stronger. It tends to be done out of curiousity from watching you strap them in and wondering "what the big buckle is for?" ;)
The only way you can help to pro long this is by making it a habit in covering up the buckle with your hand whilst doing and undoing the seat belt rather than openly showing the child how your getting them in and out each time. However this is a trick that needs to be practiced as its not so easy to do with a squirming child. I've found that by talking to the child and keeping eye contact with them, they'll focus more on your face then what your hands are doing.
If the child is able to slip out of the straps, that means they have been done up to loosely. There should only be a two finger width gap between the child and the seat belt. This may look quite tight at first but as the child relaxes in the seat they move further away from the seat belt which is why you need such a snug fit to start with. The Tobi actually has a built in two finger width measurer in the buckle so you just pull the straps until the indicator turns green.
If you like the Tobi but don't like the price (£175) you can always try the Mamas and Papas Pro-Tec (£150) which is what the Tobi is actually based on. However you will find the side impact is not as deep as the one on the Tobi.
www.mamasandpapas.co.uk/product-protec-car-seat-black-chequers/2447253/type-i/
Bebe Confort also have two good models that are worth checking out.
The first being the Axiss which has a great feature of being able to turn the seat towards the door for ease of getting the child in and out. This is a feature you may find of great assistance in a Freelander as your seats are quite high. A draw back to this seat is that there are quite a few cars it doesn't fit in so it defiantly needs to be checked for fit in your car. You can easily get this done in a couple of John Lewis and Mothercare stores. Another set back could be the price. It usually retails at about £210 which is about £60 more than the average price of a good group 1 carseat.
www.bebeconfort.com/collection-2008/UKonly/voiture_axiss.htm
The second is the Iséos TT, this carseat has the useful feature of being able to widen the sides giving more space to an older child or accomodating those big padded winter coats!
www.bebeconfort.com/collection-2008/UKonly/voiture_iseos_tt.htm
Bebeconfort are actually owned by the same company that own Maxi-Cosi so it may be a carseat brand that you've never heard of but don't be fooled, its a very strong world widely known brand.
As for getting a group 1,2 & 3 carseat, on a whole, I only would reccomend them if you don't own a car, it's a carseast thats not going to be in heavy use e.g. a carseat for the grandparents or you feel your child has outgrown their group 1 carseat but is not ready to move on to the next stage.
Having a carseat that covers all is kind off like having a washing machine with a built in tumble dryer, you'll lose some kind of efficiency along the way. Group 1, 2 & 3 carseats have very poor recline and will also take up more space in the car so its better to look for narrower models if you need to fit more than one carseat in the car. Due to the long use time 9kg - 36kg, after about 3 - 4 years periods, if in heavy use, you may find yourself needing a replacement cover for the carseat as the one you'd have been using will begin to look quite warn and shabby. All be it that replacement covers are not that expensive (about £20 - £30) their not often that easy to get hold off.
If however a group 1,2 & 3 is what you feel you need then I would recommend The Recaro Young Sport Car Seat. Its not as wide as most group 1, 2 & 3 carseats so is a good choice if you need to fit more than one carseat in your car. The seatbelt routing also fits like a standard group 1 carseat so you get a really tight fit and unlike the Britax Evolva, it doesn't require a slightly longer seat belt.
www.recaro-seats.co.uk/child-seats/young-sport.php
Incase i've confused you with talking of different groups. A group 1 carseat is a carseat that takes a child from 9kg - 18kg about 10mths - 4 years. They outgrow this carseat when there eyes are in line with the top of the carseat.
A group 1,2 & 3 carseat is a carseat that takes a child of 9kg - 36kg about 10mths - 12 years. By law a child has to be in a carseat to the age of 12 or the height of 4.5 Foot.
I hope this has help