You’re not going to find a guided holiday for £300, nothing that’s any good anyway. £500-600 is cheap, the companies that do fancy brochures are more like £800-1000 a week
If you want something that cheap, the only thing would be do sort it out yourself. That might not be too good if you’re alone though.
When budgeting, don’t forget things like getting to/from the airport (especially when you’re by yourself), things you might need that you haven’t got already and the extra cost of taking the bike. I though my first trip was quite cheap (£500), by the time I paid for the flights, bought a bike bag, spares (brake pads + tyre), travelling to airport plus parking…the costs soon stacked up. The other thing is if they’re all-inclusive. The one’s that aren’t may ending costing more by the time you’ve bought food.
Saying all that, I’ve heard good reports about Ciclomontana and I’ve just checked their prices! Much cheaper than I’d thought.
Hope you get something sorted. Winter biking in the sun is excellent!