Matching supply to demand is a tricky issue, but not intractable. It's certainly easiest to do it at build time, but retrofits are possible. Southampton built their geothermal-based heat network on the basis of the council being the first (internal) customer. West Quay, Ikea, the Dell re-development and Solent University came along later.
As far as seasons go, the trick is likely to have things like a swimming pool on the network which needs year-round heat, and can act somewhat as a buffer by saying "sure, we'll go from 19 to 21 degrees", which is quite a lot of heat in those volumes of water.
Seasonal heat storage is also a possibility. The Drake Landing Solar Community in Alberta manages to power themselves ~97% from solar, by pumping hot water into boreholes over summer and using the heat in winter. Very occasionally they have to top up the heat with gas. Perhaps most impressively, this is all off domestic roof solar as well. Sink some deeper bore holes and you could soak up a lot of spare heat in the summer.
The main thing here is that you're not architecting this so that data centres directly pump heat to homes. There needs to be an intermediary that accepts hot water, can pump it into storage, into a supply ring, or even vent to atmosphere if necessary. The actual source of the heat is transparent to both user and supplier - the intermediary buffers any mismatch.
To use the Slough example, construct a heat building, get boreholes sunk for ground storage and trunk pipes laid under the roads. As datacentres replace or upgrade cooling facilities, they can tee into the trunk and dispose of their heat (since they presumably have redundancy, and chillers are mechanical hardware with a lifespan shorter than the building - just like the servers inside). This gradual growth of capacity will match the gradual growth of connected sites - schools, hotels, leisure centres, new-build residential developments, etc. It's not strictly necessary to do a big-bang switch over. A DC could be sending some of their heat to district and some to atmosphere if they've only changed one chiller.
Given that chunks of Slough are already supplied with steam and hot water from the Slough Heat and Power CHP site (now being redeveloped by SSE), it doesn't seem like a stupid idea for Slough Multifuel to be not merely a supplier of heat, but an exchange/buffer facility, architected to receive hot water from datacentres for storage and redistribution.