Are you physically near Pram world? If not, I would look online for Axkid stockists near you as some of them also stock the Britax ERF seats and can demonstrate the fitting for you. Most/all the retailers have the same deals at the moment as it's a Black Friday price.
Tethers for seats like this come in two parts. One is a strap which connects to the seat itself, they normally come already attached. The other part is a loose strap which you attach to the car, then they have a ring and clip where they attach together.
It's the tether straps for the car which are usually unfamiliar and can seem daunting at first. If you've ever put up a tent or tethered down a trampoline, they work a bit like this, except rather than looping around a peg driven into the ground, they loop around any solid part of the car.
These are usually attached around the rails which the front seat is attached on (the bit that lets it slide forward and back).
In some cars, there is a small gap under the rails so it's very easy to install tether straps here. In other cars, it's tighter under the rails - you can normally push the tethers through with a bit of stiff card or flat plastic/metal.
If there is no access to the rails or they are too tight, it's usually possible to tether directly to the front seat itself. And another option is to buy metal loops (specific for this purpose) which can be directly screwed in to the floor of the car and then you can just clip the tether to this.
This video demonstrates: s
If you're more of a words/limited pictures kind of person this old blog post might be better, although it doesn't work online any more, you will have to copy and paste the URL into the wayback machine (wayback machine links don't work on MN): http://erfmission.com/you-cant-tether-in-my-car-debunking-myths-3/