Schools could re-open to all age groups, but the government needs to consider what's going to give to allow that.
Schools operate with 30 (or more) children per class. If you set the maximum 'bubble' size at 15, then schools need twice as many classrooms and twice as many teachers as they currently have. That's not a "relatively low level of investment", it's a huge investment in staff and temporary buildings (assuming schools even have the space!) As PPs have said, we could possibly achieve 1 metre separation with bubbles of 15, but with 30+ in a class they're practically sitting on one another's knees.
Many schools don't have extensive outside space, or multiple large communal halls.
The government have said that TAs can temporarily lead class bubbles under the direction of a teacher. Unfortunately, schools don't have TAs sitting about waiting for work to do- budget cuts have led to redundancies in most TA positions, and usually only children with 1:1 funding have access to a TA.
Additional cleaning leads to additional costs in supplies and staffing , especially where cleaning is needed throughout the day (many schools bring in cleaning staff after the children have left). Government needs to guarantee that this will be funded- currently, schools have to spend the money and then hope the government lives up to its promises (ha!) and allows them to claim it back.
Staggering opening hours/ lunch hours also has a knock-on effect in terms of costs- for example, paying kitchen staff / MDAs to work an extra hour each day soon adds up.
Realistically, the government needs to choose : a rota system where half the children are in at any time, or invest in supply teachers and temporary classrooms, or accept that social distancing is totally non-existent in schools (ie separation between children is zero metres).