Yup, in England, the seats themselves have the lock, so the belts don't.
Next time, you'll want to make sure you bring your locking clip - it's a little golden "H" clip and you carseat manual, JIC.
Using a locking clip from http://www.carseatsite.com/lockingclips.htm
- Scroll to the bottom for pics!
1. Use a lap/shoulder belt, see picture 1.
2. Feed the lap/shoulder belt through the appropriate belt path on the car seat (generally, the belt path on a rear-facing seat is under the seating area of the car seat, while the belt path for a forward-facing seat is behind the back of the car seat).
3. Buckle the seat belt in and pull the shoulder portion tight. It's easier if you have a helper do this while you press down on the seat with your knee.
4. Once the belt is pulled tight, grasp both the lap and shoulder portions of the belt right next to the latchplate and hold on tight.
5. Unbuckle the belt while still holding onto the belt next to the latchplate. You may find it easier to mark the seat belt with a piece of chalk or a pen. If you do this, be sure to mark both sides of the belt so you can tell if you've accidentally loosened it.
6. Attach the locking clip, making sure that both the lap portion and shoulder portion of the seat belt is threaded through.
The locking clip should go right next to the latchplate, no more than 1" away from the latchplate, see picture 4. An easy way to thread the seat belt through the clip is to fold the seat belt in half lengthwise, see picture 2. Slide one side of the locking clip over the folded portion of the seat belt and let the seat belt lay flat again (be sure not to let go of the seat belt; you don't want to gain any slack in the belt at all). Repeat for the other side of the locking clip, see picture 3. *Note: it's easier if you practice this first on a loose seat belt.
Once the locking clip is securely on the seat belt, buckle the seat belt again. It shouldn't be easy to buckle; if it is, you'll most likely have to tighten the belt again and redo the locking clip. If the car seat gets in the way of the locking clip, you may move the locking clip further away from the latchplate, but it still MUST stay as close as possible to the latchplate.