that you'll get plumbing fittings cheaper in a plumbers' merchants than in a DIY store.
It's not right to call it a rip-off, that smacks of something underhand, and it's not the case.
Homebase is open until 8:00 at night, Plumb centre closes at 5:00.
Plumb centre closes at midday on Saturday, Homebase closes at 8:00pm
Homebase is open on Sunday, Plumb centre isn't.
It's all about convenience shopping, long trading hours cost money, and that is recovered in the selling prices. In addition, small items in DIY stores are almost always individually packed - more cost.