Long sleeve, round neck tops are an absolute essential for layering clothes... You can also wear the polo neck types, but I find they get too hot underneath the hijab, unless you wear this style. I've put together all the shops I've found them in:
Primark: These are by far my favourites. They cost only £2.50 each, and are made from a lightweight stretchy material, so you don't need to worry about getting hot. However, they fit very tightly, so you may need to buy 2-3 sizes larger than you normally would. Available in white, black, red, grey, brown, royal blue, purple, green, yellow, black/white stripes. Only available to buy instore.
Dorothy Perkins: These are pricier at £7-£10 each, but they do have some nice striped ones.
Next: Good quality, £7 each or 2 for £12.
H&M: Made from a lightweight material, but fit very tightly. Priced £7-£8 in stores.
BHS: £8 each instore, in four colours: orange, red, brown and dark green.
Promod.com: Promod have stores all over Europe and the Middle East, but none in the UK unfortunately! Order via their website only, £8 each.
Topshop: £10 each, six colours available.
I've also bought long sleeve tops from Tesco, New Look and MK One in the past, but they don't seem to be on sale now...but keep a look out as they may become available later in the season.