Greenhouse seed sowing

I live in the South of England and have a non heated greenhouse. What seeds do you think I can sow at the moment because I'me getting impatient? I've already sown spring onion, lettuce and Brussel sprouts. Is it ok to sow cherry tomatoes now? The back of the packets are abit vague.   

Thanks for the advice.

Also another question.

My wife wants us to go away for a long weekend in the height of summer but I'm worried the stuff in my greenhouse will die without water. I've never had a greenhouse before. How many days can stuff usually go without water in summer?

P.S. You could install an irrigation system. At the simplest they are a bag of water,a nd little tubes and drippers in each pot.  Depends on how big the bag is, and how many plants, of course but it would be sufficient for a weekend away - the vent & door opening/closing still needs considering though - in mid Summer there is probably no harm leaving everything open 24/7 - I usually leave a couple of vents (I have 8 vents total) open ajar during the Summer nights - the plants aren't going to freeze in July!

This is the system i am seting up.
Full blue water barrel.
12V car window washer motor
12V timer.
Car battery wired up to motor and timer accordingly.
Pipes from the motor in the water butt then to the plants/pots in the greenhouse.
Set the timer to run the motor for 1 min at 02:00 in the morning.
The butt should last a couple of weeks just supplying enough to keep the plants going.

Things like clockwork & battery water timers (that switch off the water flow after the prescribed time) can require a minimum water pressure - i.e. suitable for Mains, but not for water butts (I've forgotten why, but I think there is a spring that is held open by the water pressure, or pushes against the timer mechanism or somesuch) ... so many/most such things won't work with low pressure supply from a waterbutt (there is, or at least used to be, a timer that can be used with a waterbutt / low pressure supply - I think it was made by Gardena).

This is the system i am seting up.
Full blue water barrel.
12V car window washer motor
12V timer.
Car battery wired up to motor and timer accordingly.
Pipes from the motor in the water butt then to the plants/pots in the greenhouse.
Set the timer to run the motor for 1 min at 02:00 in the morning.
The butt should last a couple of weeks just supplying enough to keep the plants going.

Can the gravity pressure from a water butt be used? or does it need a pump? There is a 2' or maybe 3' drop from the gutter to my water butt ... I could put some more breeze blocks under it to provide more head, and the butt itself is a couple of feet tall. But I dunno if that would actually create enough flow for some drippers or similar?

That would probably work, the dripper units that you can buy work from a bag suspended about 3ft above ground level.
The reason i am fitting a motor is so that it will only run when i want it to rather than all the time.
The butt holds 45gal's of water and if it was running constantly through drippers it may fall short of the 2 weeks that i need!
Where as if i set the flow for a specific time peiod i can control the amount being used to ensure the supply lasts the required amount of time.
45gal's may not be enough for 2 weeks so i may have to link two barrels to get the volume i need.

Trial and no doubt alot of error  

How about planting a few leeks now?

I've re-opened this topic as I'm a bit confused as to why many delay indoor sowing for so long. I sowed leeks and celeriac in mid-january and both are doing well. A speculative sowing of chillies, sweet peppers and tomatoes at the same time (during a fairly warm spell) has also been successful, with the seedlings living in lidded seed trays covered over with thick bubble wrap on colder days/nights. This last couple of weeks saw sowing of early caulis, calabrese, sprouts, beet and onions, too, and I wouldn't consider this especially early; all have already germinated. If the weather deteriorates they can be potted on until it improves again. These were all sown without heat in a cold greenhouse, BTW.

As to tomatoes, I believe we coddle them far too much and it's surprising how much cold they'll tolerate. Earlier sowings seem to result in healthy, stocky, short-jointed plants, which is just what we're looking for. Only prolonged cold and cloudy weather seems to be detrimental, but in the event that this causes failure there's always the opportunity for a later sowing.