Moving in gift

It depends where they live but I think I would try and stick to gift vouchers. Recently I gave my friend one of my watercolour paintings for her home and she was really pleased with it. Maybe a piece of local art. Also how about adopting a donkey for them, just a thought.
www.thedonkeysanctuary.org.uk/adopt?ds_rl=1249597&gclid=Cj0KCQjw6IfoBRCiARIsAF6q06sfuow8Hth613Sy7iq_xdbZXH44KoPLz1G3jwCrG0rPCr_ZcxAzdYsaAlyxEALw_wcB&gclsrc=aw.ds

One of those boot scrapers. If they've built their own house it will be muddy!

Towels! (I know I've said this before on GN, for wedding gifts, but I think that they can make a great "new home" gift, as well.) In this case, perhaps one or two pretty, good quality, hand towels - these wouldn't take up that much room? Could be squashed into your luggage?

When good friends moved into a new house (a flight away from us), we sent a few David Austin rose bushes. They still talk about them and point them out when we visit. However, I know that I would like to arrive with a gift when going to stay. Maybe a bottle of fizz from the airport on arrival and then follow it up with a delivery of a few plants for the garden after you’ve gone?

I'm not sure about number plaque. Or vase. Personal taste. I remember a friend telling me she would only ever use plain glass vases, which is not what I would have chosen for her.
If I was giving a gift token I would send it after the visit. It would be odd (imo) to give one on arrival.

I was just discussing delivered bouquets with a friend the other day. We both dislike them because they are expensive and you just see them deteriorate every day.

If I were your friend I would like a garden centre voucher so I could choose my own gift.

Do they have a number/name plaque for the new house

A book is always a good shape to stash in a suitcase. Maybe a visitors book so that they can record happy memories of their friends' visits. If there's time maybe you could get it personalised, e.g. with the new address embossed on the front.

Sorry Bradfordlass I ve duplicated your warning

Be careful with bulbs and seeds if you are flying obviously depends where you are flying to but they are banned in some countries

If you are flying to anywhere outside your own country, check with Customs that you can take seeds or bulbs - often they are banned in the country you are flying to.

If you can't buy something once you get there, a nice vase, well wrapped in all your packed clothing, might work.

Lavender bags sounds nice too.

I usually send flowers afterwards to say thank you.
When buying a present I tend not to buy anything for the home that may not be their taste. Maybe a luxury scented candle. If you have a National Trust shop close they may have something suitable.

Snowdrop bulbs are tiny and wouldn't take up much space in your bag.

If your friends aren't gardeners, maybe some lavender bags or scented drawer sachets?

I think it's always nice to arrive with a gift in hand, but I do love your idea of a delivered bouquet. I'd be tempted to send the bouquet the day before you arrive saying you're looking forward to seeing them and their lovely new home.
Afterwards might be a little like you forgot to get something. But only in my humble opinion.

Depends a bit on price maybe? Flower seeds or garden centre voucher
Subscription to a a magazine on home decoration
Gift voucher from suitable shop
Thinking these would take up no space in luggage where as a present would