It will be held at customs by whoever is handling the import. They will send notice to you of the import duty, and VAT on the cost+duty, and handling fee you owe them. This must be paid before release to you.
The commodity code for model trains is 9503003000, and there is apparently a 0% duty rate on those from Germany (hooray!).
So the payable amount is likely to be about £76.40 for VAT (depending on exchange rate used), plus handling. Royal mail charge £8 for RM products, £12 for Parcelforce. Other couriers charge different amounts, sometimes depending on the value of the imported product.
Worth checking if the quoted value on their website is inclusive or exclusive of German sales tax (which you shouldn't need to pay).