A bit late in planting the allium (depending on type)but they should still grow although may not flower this year.The dahlia should be grown on and planted outside only when the risk of frost is over at the end of may.If youre lucky and the tubers havent been badly frosted try lifting the tubers and growing them somewhere frost free before planting out.

Get the dahlias out, as Dgw says. Mine are in pots on the windowsill at the moment, been there 4 weeks for 2 of them and 3 weeks for another one. The later one is just starting to grow, the other two are well up and growing strongly - but I won't be putting them out till mid to late May.
Alliums are usually planted in autumn...

Now, maybe. It all depends. If you planted the Dahlias deeply so the base of the stem where the new growth appears, is below the level to which the soil would freeze, then they should be ok. Until the previous winter we left any Dahlias we had in the soil all year round and only lost them then as the ground froze to such a depth.
Alliums are as tough as old boots, let's face it many of the ornamental ones come from the Russian Steppes where it is a tad colder than anywhere in Wales.
If you are worried about the Dahlias then put a good deep mulch of compost over them.