Quite a lot have monthly contracts, but the bottom line is how much the companies charge for the connection fee and at the end of 6 months, the cease fee (as well as the monthly costs of course). From memory, BT charges something like 35 pounds for setup and same amount for cease...
Some ISPs charge as much as 49 pounds for setup (less for the 'cease') but as BT charges them at each end of a contract, they are able legally to pass on such charges.
A lot depends on how much traffic someone will use. If it costs (say) 70 quid (if you are very lucky) plus 6 months at 10 to 20 pounds, then that's a hole in 130 to 190 quid.
For about 80 quid you can get a Three SIM with USB dongle which allows 12 GB or 12 months use, whichever is the lower. Three also do a wi-fi unit (SIM card goes in, for connection to Three, and wi-fi signals are radiated in the house for multiple machines to connect.)
That unit is 50 to 70 quid, but there may be some deals where someone could buy the unit and a SIM for a moderate amount. On Ebay just recently I saw SIM only (with 12 GB) going for about 40 quid.
It seems to me that depending on the amount of traffic used, it could be cheaper to get a mobile SIM and share the connection if there is more than 1 PC/laptop but it will depend on where the OP is going to live as to whether Three would be usable, or how high/low the monthly charges are from an ISP.
Back to PlusNet - they offer lower rates for people in areas where there is a lot of competition (Market 3 category, as defined by Ofcom) but some of the other firms may also offer low cost high volume deals. Again, it depends on where OP is going to live.
With a postcode, it should be possible to get ideas off different ISPs, and mobile networks.
If there is a phone number (or a neighbour's number) known at the intended location, then SamKnows.com will allow one to check which alternative services (AOL, O2, etc) are available.
However, many of the lower cost deals depend on being with the ISP for 12 months. There are some exceptions, but it will depend on the exchange as to whether service can be provided.
Would be handy if OP has a phone number for destination with at least the first 7 digits of the full 11. That would make clear which BT exchange is used, and allow further checks for suitable ISPs.