If the following is a TLDR then tough but don't moan when you are cold and wet and miserable at 2am.
Nothing wrong with fiberglass poles, they can split if the tent is badly erected or in very bad weather but equally are cheap and a doddle to replace. Fiberglass poled tents also tend to have a much smaller and lighter tent bag. larger family tents with alloy/steel poles can take up loads of space. Pack size can be a pain with kids as its like mobilizing an army as is, so you either need huge boot space and a roof box/trailer or a small pack size tent.
Assuming you want an average family tent at a budget of £100-£200 then there are quite a few options. Got £400-£600 and want and easy life buy a Vango Airbeam from this season (previous seasons had some issues). With young children the ability to inflate your tent is pure win.
Which ever tent you choose for UK use you want a hydrostatic head of 3000-5000mm+. Less than 3000mm is still technically waterproof but these tend to be found on lesser quality tents so manufacturing issues like poor seams become more apparent.
1) Go Outdoors own tents (Hi Gear) are ok for dry summer use but I would not trust them in the rain having seen how many leak or have manufacturing defects on sites.
2) Coleman are ok and good value but I prefer Vango.
3) Cheap Argos/Halfords/Tesco/Adsa/Field & Trek tents are fine for the odd dry nights camping in mid summer but I would not trust them for anything else.
4) Outwell make great tents and the 4/5 mans can be had for good prices at the moment but if you damage their alloy/steel poles you tend to have to buy replacement sets which cost a fortune, sometimes nearly as much as the whole tent.
5) Vango make great tents with much easier spares supply chain and repair options. If you don't need a sewn in or zip in gourndsheet (tend to be better for family camping though), then the best value and quality family size tent for 2-3 nights camping is the Vango Omega 600XL imo. It is a technical DoE rated tent, properly water proof, very light weight alloy poles and the whole thing packs away into a 9kg small pack
http://www.gooutdoors.co.uk/vango-omega-600xl-large-trekking-tent-p261536.
The most important element of camping though is your sleeping system. A good thickness 3-7cm self inflating mat (not camp air bed) and quality sleeping bag is worth having infinitely x more than any tent. For UK camping, either a Vango Nitestar 350 or 450 sleeping bag for you and kids (adult sized is fine for kids just tuck the bottom under to fit) and a decent self inflating mat. If it is really hot in the evening then you can unzip the bag. They are really soft inside due to the lining which helps. I am yet to have a bad night sleep in the 450 regardless of temp, I have had some dire nights in other bags even high spec lightweight ones which are fine until the temp drops.
http://www.outdoorworlddirect.co.uk/vango-nitestar-450-black.html
Do not be fooled by a lot of the Blacks/Argos/Halfords/Go Outdoors own brand or special deal sleeping bags. Many will be fine for the very warm/hot summer nights but as soon as temps start dropping then it can make the whole experience miserable.
There are loads of other tent options and brands. Some cheap some hugely expensive. I like Outwell and Vango. Again with sleeping systems loads of options and my views above are just opinion. If you do buy a tent this time of year, make sure you erect it and get the hose pipe on it to test for leaks and issues.